US Conflict is an alt-history RTS where Germany invaded and ultimately conquered the US. It's a fully-fledged real-time strategy game, and it's available right now for Android.
You'll likely spend your first few hours with it working through the lengthy campaign. There are currently 12 missions to tackle, with the finale revolving around the fight for Washington D.C. itself.
You might also enjoy its extensive multiplayer modes. There's co-op play where you choose between countries including the USA, UK, Germany, France, Japan, Hungary, Romania, Czechoslovakia, and Russia. If you're more the competitive type, US Conflict also offers intense PVP action.
Each nation has 20 upgradable unit types at their command, including various infantry groups and vehicles. There are trucks, tanks, stationary guns, and much more to utilise during battle.
Knowing how to traverse the game's treacherous terrain is one of its biggest challenges, but you should also spend some time analysing the AI's behaviour.
It's wise to mix offensive play with a good defence, so it's advised that you plant mines, anti-tank explosives, and turrets in tactically advantageous positions. If all else fails, you could always fall back on a devastating airstrike.