Guerrilla warfare is a good means to defeat MUCH stronger opponents,  Basically, after you attack you move away from your location (with your move action) without your attacker's knowledge. Attacking you at that location has no effect, if you move far enough away. (Moving 10 feet to the left, for example, would protect you from being target directly by most attacks on an individual, but you would still be affected by most area effects like Fireball.)

Guerilla tactics as I think of it in AD&D normally means, in an encounter, attacking in an obvious manner, then moving away immediately in a stealthy manner and/or with the benefit of cover and/or constant motion, then repeating as necessary. Using stealth normally means your opponent does not know where you have gone or maybe not even that you have moved. Using cover in this context means that an combatant can attack and then move to a location with good cover, or some combination. Using constant motion will be describe in another post, using an example from my personal experience.

To win in a single but difficult battle, all you really need is be able to heal up and replenish combat capability and supplies faster than your opponent, for the duration of the particular battle. However, if you can avoid being affected in the first place, you can almost always do so.

Summary: be a Rogue.

It bears mentioning that, RAW, unless you actually USE the Hide action, you won't actually be hidden. So if you walk outside a fog cloud, fire an arrow, then walk back inside the fog cloud, the enemy would still be able to identify your location. This is because the Hide action represents an active attempt to conceal yourself, including moving silently.  So, if you're "hiding" in a fog cloud but you haven't used the Hide action, you're still making enough noise that somebody can pinpoint your location without an extraordinary effort. They still have disadvantage on their attack rolls, but that doesn't mean they can't target you with a lightning bolt or a fireball, or that you'd be safe from a pursuing force.

Further, if you have a fighter in your party wearing plate, then stealth is a lost cause. If you have a dwarf with a 25 foot move speed, mobility is a lost cause. If you have a barbarian that needs to make an attack/get hit every turn to maintain rage, then avoiding engagement at all is a lost cause.

That said, all characters taking advantage of terrain and cover is CRITICAL, mobile characters taking advantage of mobility is CRITICAL, stealthy players taking advantage of stealth is CRITICAL. If your rogue is standing still in a fight, they're doing it wrong. Most players forget that 5e rules allow for movement any time during your turn. So your melee rogue can move fifteen feet into melee range, attack an enemy, use cunning action to disengage, and then move back fifteen feet. Likewise, a wizard can hide behind a wall, walk out into the open, hurl a fireball, and then walk behind cover again.

good advice but not sure if saying you should move out of the way vs. just standing there really falls under the scope of guerilla tactics.

pretty sure guerilla tactics are more a form of irregular warfare for small groups to engage armies.

your last paragraph is just saying to have more actions than your opponent - which is pretty much THE deciding factor in most encounters.

One of the problems with trying to make use of guerrilla tactics is that to be effect, it requires you to pick the battlefield.  That's rarely an option for player characters.

Quote from Brian_Avery >>

Even if you're not 'hidden' due to making a stealth roll, you are still unseen inside a fog cloud and usually can't be targeted by an attack, although area of effect spells can still get you if you're in their area. It is true that you might make noise but it might not be enough to determine your location inside the fog cloud (most people aren't walking around with cowbells hanging off their necks). If some one wants to guess at your location and target that specific 5' space with an attack they will only hit you if that is where you actually are (very lucky guess) otherwise they will miss even if they roll a crit 20.

A fighter wearing plate doesn't make stealth a lost cause, just disadvantage which can be overcome. A dwarf with movement 25 doesn't make mobility a lost cause, just barely slower than average and can also be overcome. If a barbarian needs to maintain rage, he can still come out of the fog cloud and attack something close or throw javelins... otherwise yeah he will lose rage but this is not a lost cause.

That last tactic you mentioned about a wizard and the wall, could work almost as well as fog cloud. Remember if you are not making a stealth roll then any noise you make can be detected, but how loud are those noises, simple footfalls when battle is raging and armor is clanking nearby from others... good luck. An ally caster might have spells that help like Silence and Pass Without Trace or even a rock gnome using his little clockwork toys (soldier marching, beast roaring, or music box playing) might cover any noise someone makes walking quietly into a fog cloud.

No, you're cherry-picking rules from different editions there. 5e is very clear about heavily obscured people or objects:

A heavily obscured area—such as darkness, opaque fog, or dense foliage—blocks vision entirely. A creature effectively suffers from the blinded condition when trying to see something in that area.

The blinded condition, as a refresher:

• A blinded creature can't see and automatically fails any ability check that requires sight.
• Attack rolls against the creature have advantage, and the creature's attack rolls have disadvantage.

So, if you are in fog, your opponent can't see you, and automatically fails any ability check that requires seeing you, and their attack rolls against you have disadvantage. This doesn't mean they can't percieve you, as perception is based on all your senses, and they can still hear you, potentially smell you, etc., unless you're taking an action to actively conceal yourself from their other senses. I imagine an argument could be made that a Perception check to detect a character who has hidden in a heavily obscured place might be made with disadvantage, or that a Stealth check to Hide might be made with advantage, or even both. But if the Hide action isn't taken, then no, they know within 5 feet where you are (extenuating circumstances notwithstanding).

Also, 5e spells don't require line of sight to target someone, they only require "a clear path to the target," or, more specifically, "To target something, you must have a clear path to it, so it can't be behind total cover." That means, if you're blind, you can still target someone you can otherwise perceive, so long as they are not behind total cover. A blind wizard can still target someone with hold person, a cleric stumbling around in the dark can still cast healing word on their ally.

Let me state my second set of statements more clearly: In a prolonged combat situation, where one is expected to strafe and stealth over a lengthy period of time as described above, a fighter in plate renders a series of several extended Stealth checks prohibitively difficult. In a prolonged combat situation, where one is expected to maintain a respectable distance from one's enemies, a dwarf who is capable of moving at only 83% of the speed of the average enemy makes a prolonged attempt at remaining mobile prohibitively difficult. In a prolonged combat situation, where one is expected to avoid direct conflict and reduce the probability for damage, a barbarian whose core ability only remains active so long as they deal or are dealt damage is robbed of their core ability to be useful to the party. If you depended on stick-and-move tactics as your main team mechanic, a fighter, a dwarf, or a barbarian would resent the party as a whole for robbing them of efficacy, and the team would resent them for bringing the team down. If you have a group consisting of four tabaxi rogues, then this is the tactic for you, by god.

A wizard behind a wall is far superior to a fog cloud. A fog cloud is a cloud. Made of fog. That can blow away in a strong wind. A wall is a wall. Full cover trumps heavy obscurement every time, if for no other reason than because it creates heavy obscurement (rare exception would be some kind of clear wall, like glass, ice, or crystal). That said, there's a place for a fog cloud in group tactics. It's not as simple as, "Oh, we'll just pop in and out of the fog cloud and the enemy will just have to sit there and be pincushions," though. For starters, it's static. You could hopscotch fog clouds, but that's a lot of spell slots to try and accomplish what you want to accomplish. Also, the enemy can't see you, but you can't see them. And you're confined to one space while the enemy has the remainder of the field of battle available to them. So they could just say "archers, hold your actions and shoot the first person to exit the fog cloud." And then your ranger smugly rushes out of the cloud to take a potshot at someone and is turned into a pincushion.

Stick-and-move can work. but the game mechanics make it prohibitively difficult. Party composition makes it prohibitively difficult. The ACTUAL value of obscurement makes it prohibitively difficult. I'm not saying there's never a time and place for it. But it is less a cure-all and more a DEEPLY complex tactic that needs to be WELL-drilled and SPARINGLY-used.

The other issue here that everyone is taking for granted is that the bad guys play along with you. No smart enemy is going to keep trudging down the clear jungle path while you dart in and out of the brush. If you can strike and fall back, so can they. If one set of tactics can win all your encounters, it's because your encounters are all the same.

Quote from Brian_Avery >>

Both Hold Person and Healing Word do actually require a clear line of sight, you must be able to see the target in order to target the target.

And most humanoids rely almost entirely on sight to determine the location of someone, sometimes used in conjunction with hearing or smell... maybe up close in some rare cases even touch. But when sight in removed and we are forced to rely on our other senses we are almost helpless, we do not have the heightened senses of animals. We are not bats with echo-location. Trying to determine the exact location of someone within 5 feet is nearly impossible.

 Sure, for an ordinary human with 10 wisdom and no proficiency in perception.  D&D monsters and heroes are capable of superhuman feats.  Figuring out the location of a creature that isn't actively being stealthy while walking through a fog bank is one of them.  And that's assuming that they don't have some sort of extraordinary senses like Blindsight.  Plus, as has been pointed out they're typically not going to just sit there and let you peck them to death.

If you're a Circle of the Moon druid, you can engage in gorilla warfare.

I'm sorry, those are apparently both bad examples, as the spell descriptions do require you are able to see the target. But they are also good demonstrators of the point. The spells themselves require line of sight. Spellcasting does not. Firebolt, for instance, simply states you cast it on a target within range. On closer inspection, most "auto-hit" spells (spells that require saves rather than attack rolls) require the ability to see one's target. That said, you are free to target anybody with any spell that doesn't have a sight requirement in its text, whether you can see them or not.

The reason, in my opinion, that being heavily obscured is not the same as being successfully hidden is because they created the Hide action in the first place. And they made it an action. A 20th-level fighter can make four consecutive attacks, or they can hide. There's weight to that. You are free to interpret the rules however you like, but per RAW, being heavily obscured isn't the same as being hidden. Heck, by RAW, being heavily obscured doesn't even impose a penalty to Perception checks. Being hidden means you are masking the sounds you make. Without it, leaves and twigs break underfoot, your bootheels click on the cavern floor, your breathing is labored from exhertion. Your armor rattles and creaks.

We all think "how could someone hear me in the heat of battle?" But we wear cotton shirts, skirts, dresses. Denim pants. Rubber-soled shoes. Even in these supremely silent clothes, we make a thousand thousand tiny sounds to give away our location. Imagine you're clad head to toe in leather and steel. Imagine you've got a sword at your hip. Imagine you're running through the woods, or a cave, or a gravel road. Now. Still your breathing. Pay attention to any part of your body that might make noise. Every part of your body.  Keep all those parts perfectly still. Pay attention to everything around you that might make noise. Now do it with your eyes shut. Now do it while running.

If you're not spending every part of your attention doing that, how on earth do you think you'd be able to conceal your exact location from anyone?

Fog Cloud creates a fog - things within are considered heavily obscured.

A heavily obscured area—such as darkness, opaque fog, or dense foliage—blocks vision entirely. A creature effectively suffers from the blinded condition when trying to see something in that area.

Chapter 8 - Adventuring - Vision & Light

When trying to see something in the fog, the target cannot be seen, as detailed in the rules.

Unseen Attackers and Targets

Combatants often try to escape their foes' notice by hiding, casting the invisibility spell, or lurking in darkness.

When you attack a target that you can't see, you have disadvantage on the attack roll. This is true whether you're guessing the target's location or you're targeting a creature you can hear but not see. If the target isn't in the location you targeted, you automatically miss, but the DM typically just says that the attack missed, not whether you guessed the target's location correctly.

When a creature can't see you, you have advantage on attack rolls against it.

If you are hidden — both unseen and unheard — when you make an attack, you give away your location when the attack hits or misses.

Chapter 9 - Combat - Making an Attack - Unseen Attackers and Targets

You therefore don't need to make stealth checks to receive protection from the fog, darkness or whatever. As Brian_Avery pointed out: the attacker will have to guess where you are and make an attack with disadvantage - if you're not there, it auto-misses. This happens whether the target was stealthing or not.

Beyond this it's a DM's call.

-

This thread mentions Guerrilla Warfare but only refers to one specific type: skirmishing (attack then run/hide, repeat). Perhaps consider the other types in your discussion like traps, ambushes, surprise, disguises, snipe builds, infiltration, etc.

That is not the way Fog Cloud works. You know the square they are in, you just have disadvantage on your rolls. Heavily obscured only does what it says it does in the PHB, nothing more. You don't need to guess a target's location unless you can't see or hear it. You can argue that combatants are just naturally, automatically silent but it strips a lot of power from things like Silence, the deafened condition, and most egregiously the hide action, which is not easy to do for a reason.

You need to hide for creatures not to know what square you're in. That's what the rules say. Hiding provides a significant bonus beyond heavily obscured. That's why it has restrictions and why it's a full action.

I don't want to wade into this too far, but Jeremy Crawford has addressed invisibility and hiding extensively.

https://dnd.wizards.com/articles/features/james-haeck-dd-writing specifically starting around 25 minutes.

Quote from Brian_Avery >>

Thank you but that link just went to some game or other and had nothing about Jeremy Crawford addressing Invisibility.

 I don't know what to tell you. There's an embedded podcast on that page in which Crawford talks about Stealth for more than 30 minutes. Maybe look again?

Edit: for anyone that just can't even https://media.wizards.com/2017/podcasts/dnd/DnDPodcast_04_27_2017.mp3 Crawford's segment is from 8:48 to 47:58.

First, as the original author in this thread, I am somewhat encouraged by the fact that I have not seen anyone here addressing the morality (not ethics, that is evil vs. good) of guerilla tactics as opposed to other types types of tactics, in terms of fairness in game play. It sounds like, as players, you are willing to work that out by means such as trial and error and discussion in general, rather than unilaterally declaring that it should not be done on that basis.

This thread was partially inspired by a 4e encounter description of the party being ambushed by a wizard with Fly and Improved Invisibility (the 4e version of Greater Invisibility) in effect. While the description did not refer to guerilla tactics or to the wizard moving from place to place on his move action after casting a spell, I considered it possibly implicit, as it was in the context of providing more difficult encounters for the PCs. The wizard would presumably have been at appropriate distance for spellcasting like Fireball and Lightning Bolt but not returning arrow fire.

(Unfortuneately, the link to Jeremy Crawford's article did not lead to any article at all for me.)

Most of the comments are about the use of concealment, not of cover or motion. The discussion about Fog Cloud seems too focused to me, given the original subject. Other relevant situations do apply, such as magical darkness, being underwater in murky water or just deep, dark water, and the like. Some such as the darkness of night and a few others were referenced. How about smoke or fog that is continuously genererated, such by a forest fire, an Eversmoking Bottle, a pile of trash being burned nearby, or the weather (thick fog)?

There was at least one reference to the use of blindsight, and that is a good thought. (An attacker would not need to move out of the fog in the first place, if the target is within the range of the blindsight.)  Tremorsense and blindsense, without precise targeting, seem implicitly referred to.  Also, there does not seem to be a limit in 5e's version of See Invisibity's distance at which invisible and Ethereal creatures, although there is for True Sight. And, in 4e there were a few descriptions of magic abilities that allowed vision to not be hampered by smoke or fog. Given that some modern technology does do the same thing, I think it is reasonable within the game.

I've been thinking about this thread most of the day and the thing that I think is being missed is that broadly speaking, asymmetrical warfare is what combat in D&D mostly looks like.

If the PCs are out to destroy Count Dorkula, their typical move is not to waltz up to the front door of his castle and try to take on all of his forces in a single protracted battle.  Most of the time, they're going to try something more like finding an infiltration point that's not being guarded, then head through the castle and pick off smaller groups of enemies in short engagements before  confronting the big bad.

Quote from 6thLyranGuard >>

I've been thinking about this thread most of the day and the thing that I think is being missed is that broadly speaking, asymmetrical warfare is what combat in D&D mostly looks like.

If the PCs are out to destroy Count Dorkula, their typical move is not to waltz up to the front door of his castle and try to take on all of his forces in a single protracted battle.  Most of the time, they're going to try something more like finding an infiltration point that's not being guarded, then head through the castle and pick off smaller groups of enemies in short engagements before  confronting the big bad.

 That is the usual go to strat, except when the characters muck up a critical roll and have everything go sideways. 

When I first read your post I was thinking of guerrilla tactics for the party’s foes, not the party themselves. Play your foes like they want to survive as much as the party does and you’ve got a whole new game. 

Quote from Greg >>

When I first read your post I was thinking of guerrilla tactics for the party’s foes, not the party themselves. Play your foes like they want to survive as much as the party does and you’ve got a whole new game. 

It was in fact the tactics used by the PCs I was thinking of. (The mechanics of the game make it difficult in most circumstances.) Getting all the players to work together on it can be impossible, but a low-level character can really shine this way, and even the DM might not realize you are doing so for a while.

Most players do not understand such tactics well. In my experience, getting slammed by low-level opponents in unexpected ways can make the players feel like the DM has changed the rules in an unfair manner. If they do not already have at least a basic understanding of guerilla tactics, it should be explained to them after the adventure or as part of play, or they will be upset with the DM to some extent.

6thLyranGuard, I personally do not know what "asymmetrical warfare" is. I was infantry, but not an officer, and do not know if officers would know the term.

There was no intention of implying how the PCs would choose one particular method of achieving a larger objective than one encounter, although avoiding guarded entrances is better if possible. (The "no adventure happens if you don't get through the main gate" attitude does often apply in most game sessions, though.)

Asymmetrical warfare.  Basically, it's the technical term for when two sides in a conflict have dramatically different levels of strength, making head-to-head fighting a default losing strategy for the weaker side.

In most games, the player characters will be individually powerful but not so strong that they can take on the entirety of the enemy's forces at once, thus they use ambushes and stealth to reduce the enemy's numbers down until the leader becomes vulnerable.