Well since America didn't exist until AFTER the American Revolution, it would have to be the War of 1812 when the British attacked and burned the US Capitol building and the White House in Washington D.C.
http://wiki.answers.com/Q/In_w...

...

Well since America didn't exist until AFTER the American Revolution, it would have to be the War of 1812 when the British attacked and burned the US Capitol building and the White House in Washington D.C.
http://wiki.answers.com/Q/In_w...

There are other answers depending on what technicalities you wish to observe.
Poncho Villa also crossed the New Mexico border and attacked the town of Columbus in 1916, which caused the "Punitive Expedition" by the United States. Pershing's expedition is in the War/Police Action gray area,

Add WWII. Pearl Harbor was US territory. There was also a small Japanese invasion of the Aleutian Islands.

September 11, 2001 War requires a first act of aggression to occur. In my mind 9/11 was an act of war on the United States.