The US have always been seen as the invincible nation, the one that can never be invaded. Modern America has never been invaded before. This subject was much talked about during the 80s with the cold war, and is a popular subject of any Tom Clancy like novel. Of course, video games have also explored such a possibility.
What would happen if the US were invaded in real life ? It only happened once during the war of 1812 where the British Empire occupied Washington D.C. This led to the Burning of Washington with the destruction of the White House.
If this were to happen today, the President and the executive branch would be moved to a safe bunker. This bunker could be the one near Colorado Springs, Cheyenne Mountain. Nicknamed Cristal Palace, this bunker not only houses much of NORAD and a Combat Operations Center, but also has blast doors that can resist the detonation of 5 nuclear weapons. If the president were to die, the Secretary of Defense would take over. If ground control is destroyed, operation Looking Glass comes into play. This consists of a U.S. Navy Mercury jet that takes control of America's nuclear forces.
In Video Games
You as the player must of course save your country from the invaders. Such games with these kinds of scenarios usually have you running around important monuments like the White House and evacuating key members of the government. Some games like Freedom Fighters have you play as the resistance against the invader's rule. Some invasion scenarios are the result of an alternate time line (like in Turning Point: Fall of Liberty) or happen in the near future (like in Modern Warfare 2).
Such a plot is usually ends up with someone firing a nuclear bomb or some iconic monument being destroyed. The idea of America being invaded is almost impossible in this Modern era. NATO and the U.N. would never allow it as such an invasion would likely result in nuclear war.
- Modern Warfare 2: after a terrorist attack conducted by Russian ultranationalists which frames the US, Russia invades America by attacking Washington DC.
- Freedom Fighters: In an alternate timeline, Russia ends World War 2 by dropping an atomic bomb on Berlin. It becomes the world's only super power making all of Europe switch to communism. They decide to invade the US as to "liberate" the nation.
- Resistance: Fall of Man: the US is invaded by aliens called the Chimera in an alternate timeline where World War 2 never happened.
- Turning Point: Fall of Liberty: In an alternate timeline where the Nazis win the second world war, Germany invades the US on both the west and east coast.
- Fallout: China occupies Alaska triggering the Great War.
- Command and Conquer: Red Alert 2: The Soviets invade the United States as a means of revenge for their involvement in the Great World War II.
- Command and Conquer: Tiberium Wars 3: Nod forces launch a series of suprise attacks on Blue Zone locations in the United States following the destruction of the Philadelphia.
- Call of Duty: Ghosts: Set in a future where a South American superpower known as "The Federation" has invaded the US.