Мouse to aim and shoot.
Press 1-3 to select different guns.
Press 4 to select a grenade.
Use guns to shoot down troopers, choppers and armored vehicles. Use grenades to kill landed soldiers. Four soldiers from one side will bomb the gun.
Eliminate the paratroopers invasion by shooting down paratroopers, choppers and armored vehicles. Don't let them land, or your gun will be bombed.