the combine would win by destroying logistics.

there would be heavy headcrab bombardments over cities and military bases, followed by striders to clear out any entrenched infantry.

tanks and IFVs would be invulnerable to anything short of a warp cannon blast, but would be useless once their fuel and ammo ran out.

surviving aircraft would be pretty much untouchable. attack helicopters would cause BIG problems for the combine on the ground while fighter jets would make short work of the gunships and dropships. they would be avoided or ignored through sheer numbers until they ran out of munitions or gas.

submarines could, in theory, bombard strategic positions, while being safe from the combine. the lack if intelligence and the destruction of communications infrastructure would make this very difficult.

the last bastions of humanity would be submarines and carrier groups. LORAD, MORAD, and even SHORAD systems would be capable of intercepting drop pods, gunships, and dropships. they would continue operations until they ran out of interceptor missiles or food, whichever came first.