I am writing my next adventure and decided on a nearly pure combat adventure with little role playing, intrigue or exploration.  An army attacks the city, which the characters are invested in.  The light infantry would be something wimpy like orcs with a few giants serving as the heavy troops.  The invaders’ goal is to plunder and kill-very basic. 

I thought the light infantry could all be one shotted, making the guys feel like heroes.  But there would be tons of troops to attack, like a swarm of bees.  The PCs may decide to leave the city, but would have to fight their way out. 

 If they decide to stay and fight, I need help in deciding how to run the adventure.  Could they sway the entire battle in favor of the town?  I don't know how I would run the battle between the town guards and the invading army, especially if that is happening simultaneously with a PC battle.  I have read suggestions on mass combat but have never run one myself.  They could use siege weapons, like ballistas, if they were clever and made a stand from a defensive fortification but this would add more complexity to the battle.

 I like this idea because it is not me and I have never run an adventure like this.  Any ideas would be appreciated.

I would think of a mission for the characters to have to attempt during the battle. Maybe the baddies are attacking the city to get a hold of a McGuffin and the players are tasked with its safety, either defending it or escaping the city with it. 

Or maybe they're trying to repair the city's magical defenses to repel the attack, but doing so means having to fight their way in a small formation across the city to different locations to connect A to B and get the tools to patch up C etc.

Not everyone's job in every battle is just "reduce enemy forces to 0", so if you think of the strategy the attackers or defenders might be using, you might be able to come up with something specific your players can do as part of a greater overall plan to prevent the enemies from doing their thing, or the plan to execute the defenders thing. 

For big climactic events like this, for the party to turn the tide they will need to perform some great act - kill the enemy general, awaken a sleeping dragon, that sort of thing.

I like to pepper the world around this sort of thing in "options" for the party. For example, I had my level 8 party fight an adult-ancient dragon (adult breath weapon, ancient health). Building up to the fight, I highlighted the large ballistas on the roofs of the town called Hashbows, the giant water tank suspended on the cliff above the town, and an automated slaughterhouse which ran on iron golems, told to slaughter anything which was large and had 4 legs.

The party learnt all this before they even left to anger the dragon, so when it followed them back 8 sessions later, they knew to man the Hashbows and try to lure it into the slaughterhouse. It worked; they felt awesome for using it, I felt awesome for it being used, and we all sat around the table in mutual awesomeness-feeling for a short while, with me telling them it was awesome they thought to use it and them tellimg me I was awesome for foreshadowing it.

So, depending on how far off this siege is, you can establish some groundwork:

1: There is a farmer to the Northwest who is in town claiming his sheep were all killed by a pack of Bulettes, which are roaming his land now and won't leave. Players could coax them to the fight.

2: There is a large, rickety stone tower (condemned) on the edge of town which many fear will come crashing down. It could be pushed outwards onto the enemy.

3: An NPC has recently discovered that the western edge of the fields is infested with large worm creatures which seek out ale. He might be a lunatic, or you might take out half the army by launching ale barrels at them.

That sort of thing, foreshadowed and left for them to use or not use as they desire, can make this sort of scenario so much more rewarding!

I have run sessions in which there was an army attacking a city and the number of enemies was overwhelming and they grow tiring quickly. Some of it was inexperience as DM but another is that you need something to engage the players.

I haven't run it but Beyond the Dragon of Icespire Peak: Divine Contention has a chapter in which a town is besieged and so does tyranny of dragons iirc. I would also advice to look up rules to minimize rolls or group up opponents so that it doesn't take forever for the players turn to come in combat or to find a way to keep them involved during the opponents turns. Keep in mind that 5e is somewhat geared towards short encounters so you should plan for a way for the players to recover resources, like helpful clerics, potions, arrows, etc. Also find a way around the class shortcoming for this kind of encounters: martial class can feel sort of underwhelming against a lot of enemies when they compare their 2 attack per round to the spellcaster area attacks when fighting minions unless you use optional rules for cleaving attacks. Similarly, spellcasters will run out of spellslots fast in a marathon run for a combat focused campaign.

So, one of the things about running an open world is that this can happen.

I have had players besiege a city. By themselves. At high enough levels, they absolutely can if they are creative enough and you didn't plan for it, lol.

I have also run large scale battle several times. You have to organize your invading army on the basis of squads or teams, first off. Determine ranks, all that. The reason for this is not role play or even something for players to know -- you do it because you are now the general commanding the forces, and you have to operate like one.

you will send in squads to dump on trouble spots, and that's why you need to know.  You'll have to have an idea of their damage in both melee and at range (and if they don't have ranged combat, then don't even bother).

Ranged units are usually behind the lines, firing into them. That includes their mages. For an army, figure out how you want to do melee set ups -- there are optional facing rules, but I usually stick to 5 to 8 at a time max, and then I use the Cleaving Rules. Normally it is groups of one to three, and I will indeed roll it as normal. I presume there is someone behind them so I don't have to deal with backside attacks (if not, well, then that's the 8).

A Hex system is going to give you 6 directions, a Grid is going to give you 4. With "Size and Space" rules, that's your "rule based" limits (I use a grid and allow corner attacks).

Take out the general, and the Army will begin to fall apart until a new general is raised, essentially.

I will say one thing, however: this is not a short, brief, fight style ting -- there is no break in fighting except as the end of a day. In a standard field battle, fresh troops come up every two to four hours, with orders to retreat to any still standing, and that's so they can be bandaged. 

Most major battles where both sides care about their wounded will end a couple hours before sunset, so that the armies have a chance to collect their wounded. Small Skirmisher units will be put out by both, and mercy blows will be frequent. 

With typical D&D bad guys, that might not be the case, however -- they might not honor traditional customs, or give a damn about their wounded. But all armies need to eat, need to drink, need to rest, need to tend to gear and equipment, need to regroup. All of them. So they will have to stop sometime, and up to the general to figur eout when.

Use fatigue rules heavily, if you want grit. otherwise, yeah, use regular short rest and long rest rules, lol.  Fresh as the morning dew each morning to go out and face equally fresh as the morning dew bad guys.

War of this sort is a grind. It doesn't let up. The actions oof the PCs, if they ar e heroic (they cleave through three foes, they leap over the heads of the enemy to take down the squad leader, they hurl a mighty fireball that takes a chunk out of the oncoming line) will spur morale of the troops around them, but when they get hurt, well, that makes the morale plunge.

So everytime they take a lot of damage (from a spell targeted on them, for example) or they have a fumble (and you will be rolling a LOT) everything moves back six feet (the linee wavers), while if they have a crit or do something really cool, it moves up six or 12 feet. That makes them the drivers of the battlefield.

Don't explain it to them, though. Just say things at the end of each round like "the line moves back, the line moves up, the line doesn't move".

Really beef up the carnage aspect, though. The smell of blood and stench of loosened bowels, the sight of crushed bodies and severed limbs, the open vacant eyes.

Now, all of this does require that you have an idea of how many troops there are on each side to start with as well. If the bad guys drop below 20% of their troops, they will flee (or whatever number you choose).

At the end of each round, roll a die. if you have like 1000 people on the defender side, roll a d20 or d30 or d%. That's your losses. Double the kills of the characters for the the attackers, and that's their losses.

You go back and forth until one side is overrun or surrenders/flees.

I should note that the rules I sorta explain (probably poorly above) are the rules I use in any situation where PCs enter a "battle". It is a system that I came up with about seven years ago or so? that worked really well and has been used frequently by our group for such things.

I know that a lot of folks want to do the whole PC's command legions and all of that, but for that, I sorta think one should be doing more of a wargaming style, which isn't fully conducive to D&D style play -- there's no roleplay to it, and the characters fade back.

So this rather very simple system allows for Players to walk into a war and probably die (don't even think it isn't likely, lol, but after six or eight turns they will be thinking "oops") but if they don't, they can turn the tide, they can shape the battle, and they can feel like heroes.

In my next campaign, I have an area where there is an ongoing and rather constant war -- weather permitting. It is a meat grinder, a horror show, and if I ever do a "war genre" adventure, that is where it would take place. It provides me with the ability to give background information and to have a sadness in some places over the loss and even a topic of inn discussion -- it is a background feature that every year, thousands of people die, and hundreds return, hurt and disabled and scarred inside and out.

Of course, the bad guys have it worse, lol. 

I am working on goals for the battle.  I will try to be succinct at the risk of not giving you enough details to follow.

In the last adventure, the party discovered a merchant had unleashed a plague on the city because he had procured an artifact that healed any disease.  (The disease was otherwise nearly universally fatal).  He charged the sick large fees to heal them.  Eventually it was discovered that he was also the source of the plague (like the mafia providing "protection" from their own thugs) but he escaped the city, leaving behind the artifact and the charmed priestess he was using to heal the populace.  The artifact went to the chief priest of the goddess of love and healing.

I was wondering how the merchant got the artifact to begin with.  What if he stole it from a dragon's horde and the dragon discovered what happened and now wants it back!  If the dragon sends a bunch of minions to steal it back, who raid the merchant's manor.  When they don't find it, what would happen next?  Would the chief priest turn over an invaluable artifact to pacify the dragon and raiders?

This sort of battle would also be easier to run as you are talking about dozens (not thousands) of monsters and NPCs.

Thoughts?

This is what I have so far.  Any thoughts?

-Avner the merchant, hired a thief to steal the Tears of Ehlonna necklace from Saphira the Green dragon’s horde.
–Saphira figured out who stole it (captured the hired thief and tortured) Wants vengeance
-Bumbling assassin sent by Avner
-Small goblin raid at night ransacks Avner’s old manor in the north part of town to find the necklace.  Unsuccessful as Avner has fled and the temple of Ehlonna has reclaimed the artifact.
-Serious assassin sent by Avner
-Ambassador sent to the Duke and demands the return of the necklace and Avner in chains

-A) Refusal results in an all-out assault on the town by a goblin army. Army centers attack on the temple of Ehlonna and takes the necklace from Father Mathis.
The town is mostly laid to waste by the dragon.
PCs lose money, friends, etc and will want to hunt the dragon down next time

-B) Agreement results in a 30 day truce, to return the necklace and procure Avner.
Avner is far away, living in a farmhouse.
Has decent supply of money to hire assassins and dreams of rebuilding his empire
If caught and turned over to Saphira, dragon still destroys town
PCs lose money, friends, etc and will want to hunt the dragon down next time

After completion of paths A or B, Father Mathis is willing to help the PCs kill the dragon and recover the necklace.

Sounds good, be sure to leave paths for possible additional actions you can’t predict, but it gives a good strong set up.

Quote from AEDorsay >>

Sounds good, be sure to leave paths for possible additional actions you can’t predict, but it gives a good strong set up.

 Of course. My party never does anything predictable