>>14304258
I'm assuming you're asking for an FPS entry for this due to the other examples posted, so I'll ignore the obvious ideas of making it RTS or RTT.
Those would actually help mitigate the issues there and would let it be somewhat easier to do as well.
The US side is standard base defense in first person. You stay in a bunker or some other fortification and you shoot the rampaging hordes as they come near. Plenty of games already do this and they are quite fun as well. However considering your Claymore mention, I'd suggest a "Orcs must die!" aproach. Players can not only shoot at the advancing horde but also setup defenses. Forward fox holes, entrenched machine guns manned by NPCs and even set up mines all around.
Waves have a small interval in-between them where players can do so freely.
The Korean side is indeed the hardest part to solve since it happens in open ground, but I'd like to say it once again: "give every player a squad of NPCs that follow him and his orders". It's something that pretty much every FPS that tries to portray large scale fights should get.
Every Korean player would instead have a squad of 5-10 soldiers that follows him around. He has a 3rd person view of the entire squad while they charge, controlling no one in particular but as soon as he tries to aim, fire or gets close enough, control switches to one soldier in first person.
Being killed will see the camera swoop a bit up and then back down on another soldier in your squad in a slow process that allows a player to get his bearings, until you no longer have squadmates.
Every wave would have a certain time of 5 minutes, after which an interval of 30 seconds for the US would let them replant defenses before a new wave is sent.
The first team must hold out 3-5 waves, then they switch sides and the oposing team is now defending.
Alternatively, every round begins with 30 seconds of setup and proceeds to 5 minutes waves, with teams alternating sides every round.
However, your ammo and supplies (that you spend to build fortifications) do not replenish at all as the game goes. The first team to lose their base (likely because they ran out of supplies first) loses the game.
There can even be an interesting mechanic where Food is a resource that affects walking speed and health with larger armies consuming more.
So a side that is clearly winning with no casualties will run out of food first, balancing the odds and forcing them to take the oposing base faster.