What happened when the British troops left the American colonies?

Last night I went to a reception for Sami Rasouli. He was asked what would happen if U.S. troops left Iraq immediately. His response began with the line above and continued:

What happened when Ghenghis Khan left Iraq? What happened when the Turks left Iraq? What happened when the British left Iraq?

The conclusion is obvious: you have to remain totally ignorant of history in order to believe the U.S. troops must remain in Iraq. Unfortunately, the majority of Americans never learned this history.