my players want to lead an army and want to do some army on army fighting any ideas on how to have it done?

This is a question that comes up periodically, the simple answer is not easily. By it's nature DnD is a skirmish game, there are other game systems that allow you to run this kind of large scale battle. 

Having said that there are a couple of approaches you can take. 

Have the players as generals, in this case make it a contested skill test challenge vs the enemy generals, the higher score wins and shifts the battle in a particular direction but it can be really dull. 

make a map, cut out bits of paper/use minitures, to represent units and  create a host of rules to "fight the battle" 

Explain to the players that the reality of war in a fantasy environment like DnD is that once battle is joined they will be stuck in the middle of it, fighting the enemy in front of them so losing sight of the overall, in this case you can setup a number of encounters for them to deal with success shifts the battle in their favour. 

But it is worth just setting your players expectations, what they think they might get out of this session may be very different to what you can actually deliver within the limits of DnD rules. 

https://gamenightblog.com/2021/01/31/fantasy-siege-roleplay-combat-resource-how-to-run/

This has some great info on running sieges, this could be dramatically expanded for a war.

Matt Coleville has a book coming out called "Kingdoms and Warfare" that will be all about simulating war in D&D. I think it is due out later this year.

I worked up some mass combat rules awhile back for my homebrew campaign. They are by no means perfect, but it worked fairly well for the session I needed them for.

https://drive.google.com/file/d/1mahfnWv4WPdhZI1uliqRwZE2xn5os23w/view?usp=sharing

I always dread these. To avoid having too many characters on the board, I usually just have the players be a squad that's flanking around to confront the boss while NPCs duke it out on the actual battlefield.

Quote from KurtalmakTheKoboldKaiser >>

my players want to lead an army and want to do some army on army fighting any ideas on how to have it done?

 

 Not sure, but might be a way you could do it on https://dndbattle.com if you import the dndbeyond characters.  You can only increase the map size to 24x24 or something like that. Also its not like a vtt, it will run the characters and monsters and show the results but maybe you can figure out something useful from it.

I did that for the end of my last campaign, it really wasn't that hard. Simply treat them as swarms. One swarm for the human archers, another swarm for the Wereboar Barbarians, another for the cavalry on horseback.

I don't mind a bit of math during prep, so I calculated what the chances were of each 'swarm' hitting each other swarm, and what their average damage would be. HP = total hp of all contents of the swarm, and AC is AC. I then multiplied the chance of hitting against the average damage to eliminate dice rolls. Once the swarm reached half its HP, its damage is reduced to half (just like any other swarm in the game).

If Swarm A attacks Swarm B they inflict 30pts of damage - no roll needed.

If Swarm A attacks Swarm C they inflict 42pts of damage, etc.

So, my advice, is to break up both armies into groups and make those into swarms.

I think a good way to run a war is to have the characters fight the powerful enemy general or champion on the field and every so often have the characters troops run on and sacrifice themselves fighting the big guy to give the characters time to heal or to take a hit, and have some weak enemies run on to get demolished and boost moral. Adding a table of events, like a cavalry charge or volley of arrows can add a dynamic element, and describing how the forces are doing against one another adds to the enviornment. 

Your mileage may vary but this is how I handled it myself.

Core Rules of Army Fighting

• Armies are divided into commanders, infantry, and fire support.
• Infantry and Fire Support are units (swarms) of 50 individuals.
  • The Unit size is Huge (20ft x 20ft) 
  • Group HP (500), with 1 individual dying for every 10% hp lost. 
  • Units are commanded by a commander. If the commander dies, or flees, the unit will always attack the nearest enemy.
  • You cannot target an individual within the unit (swarm rules).
  • Infantry & Fire Support can only target other units (swarms).
  • When defending weapon, spell, or abilities that target just one individual can deal a maximum damage of 10hp per attack.
  • Grappling, shoving, and other such abilities cannot be used against a unit (swarm).
  • Spells with an area effect (e.g. fireball) which target the unit deal their usual damage but the unit (swarm) make only one saving throw.
  • Attacks
    • Infantry - When attacking another unit (swarm) within melee range (5ft) have +5 to hit, and deal 5d10 (bludgeoning, piercing, or slashing depending on armaments) on a hit.
    • Infantry - When attacking a commander, or player character within melee range (5ft), always hit an deal 5d10 bludgeoning damage on a hit.
    • Fire Support - When attacking another unit (swarm) have a range of 60(120)ft. They have +5 to hit, and deal 5d10 piercing damage on a hit.
    • Fire Support - When attacking a commander or player character have a range of 60(120)ft. The have a +15 to hit, and deal 5d10 piercing damage on a hit.
• Commanders are the equivalent of player characters. They each control and command a unit of either infantry or fire support.
  • These are usually 'normal' Monsters or enemy stat blocks.
  • They always attack at disadvantage against Infantry or Fire Support due to their sheer numbers. 
  • When they reach 1/4 of their max hp, the commanders make roll a d20 for courage.
    • 1-5 - The commander, and the unit (swarm) they command flee the battlefield.
    • 6-10 - The commander flees the battlefield.
    • 11-15 - The commander attempts to withdraw (dodge, or hide) from combat, but remains on the field to attempt to command their unit.
    • 16-20 - The commander fights to the death.

Initiative is basically handled as 'normal' combat. Each player rolls initiative for them, and a flat d20 for the unit they command. GM rolls initiative for the commanders and the enemy units. So, in a five-player game there are five player characters, five player controlled units and then whatever the enemy force consists of. If you want an evenly matched battle then that would be ten enemy units. The overall aim is that players should be instructing their units to attack other units, and focus their own energies on taking out the commanders.

It is  FAR from a perfect solution, and D&D really isn't built for combat with armies, but it worked for me on four out of five occasions that I used it. The fifth group felt it would be better if they could target individuals within the infantries and fire support units. Personally, I disagree, but to each their own.

3le is being fought and eliminating as many of the leaders as possible. let the war/battle be the backdrop to their actions. Take alook at Tyranny of Dragons for how they did it. its probably a better idea than trying to homebrew being the generals of one side. 

Dragonlance: Shadow of the Dragon Queen has several mass combat events that are tied to the board game so rather than using PC's you can play that to simulate how the main battle is going.  The Board game is kinda fun on it's own but I don't recommend doing it just to do it.

In my other hobby I do 15mm miniature combat.  The rules are simple and fairly easy to explain/ use.  The system is Black Powder (which is designed for Napoleonics) and they also have a system called Pike and Shote which is for early gunpoweder era.  You could change from DND to something like that for the "mass combat" giving some kind command bonuses to each character's lead units based on some basic skill checks.  Maybe if the player succeeds on a "command roll" the unit gets an extra inch of movement, or an extra inch of range.

Then, when things start to get "heavy" you can do a "Zoom in" and go from the mass combat game to a close quarters Heroes vs Heroes moment.  

DND started out that way really.  GGygax and Co wanted to play miniature combat but rather than focus on legins, focus on small numbers of heroes doing great deads.  Less Rohirm smashing into Uruki, and more Witch King sparing with Aragorn.  No reason you can't go back a bit.

Yeah, it's usually just easier to run war like a backdrop while the characters do the important things.