Looking for ideas on how to handle "off-screen" NPC combat.  PC's will be essentially in a war, a unit of helpful NPC's will break off to handle another threat, while the PC's take on the BBEG.  I was looking for a way to simulate that side battle without actually conducting the full turn by turn combat, but still having the results feel real and natural, rather then deus ex machina-esque predetermined outcome (i.e. length of combat, damage taken by the NPC's, potential casualties, etc.).  Especially since the idea is that if the BBEG battle takes long enough some of those NPC's would theoretically be able to help the party, assuming they survived their combat.  Any ideas?

Do it beforehand and keep notes about what happens to who and in which round. You can even pepper in descriptions of that fight during the PC fight, and you’ll know which round the NPC’s would be available to come and assist. 

Whatever you do, don’t do it during the session. Nothing is more boring for a player than sitting there watching the DM roll dice about stuff that won’t impact them. 

Mob rules in DMG ch8 would work decently if you don't mind there being no RNG.  Each round both sides would do their average damage to each other and remove that many combatants from the field.

Just narrate what is going on in the background as no-one wants to sit and watch the DM rolling dice by themselves for NPCs to hit each other!

I had a small arena tournament where the party won their first fight and then they had to watch two other teams fight to see who they would fight next. The fight was predetermined by me so I just did a one-minute narration that gave them a little bit of useful information about the combatants, how they fought, and what kind of trickery they had up their sleeves.

Thanks for the responses.  I definitely didn't want to conduct the combat love in front of the players.  Not onok y would that be boring for them, but it would not be too exciting for me either.  I guess I should have clarified that was not what I was going to do.  I was more looking for a way to expedite the combat during my prep, so I wasn't conducting full combat by myself.  Thanks.

I once played in a party a long time ago where something like this came up. This was back in AD&D times, but it's a simple and easy to use mechanic that works well in any game version. 

Each round, at the end of their turn, each player makes a die roll, and so does the DM.  The number the player rolls is how many enemies their group of npcs kills that round. The number the DM rolls is how many members of that npc group are killed by the enemies that round. Also - a player may choose to forego their attack against the Boss by instead attacking the minions, in which case they make the same dice roll but double the result. 

So let's say you've got a party with 5 characters with 200 allies in the assembled army. And you've got a BBEG with 250 minions. And let's say you decide the die roll each turn will be a d8. So each PC is responsible for a group of 40 allies. Each round at the end of their turn the player rolls a d8 and that number of enemy minions are killed, and the DM rolls a d8 and that number of members of that ally npc group are killed.

It's just one extra die roll each turn, and it actually makes it really easy to track the attrition of the armies, once you get the knack of it. 

Just my 2 cp. 

Simple and elegant with lots of room for players to add their own flavor. 10/10 will use.