Many Americans don't realize that Canada was in WWII about 2 years before we were.
But, in terms of knowledge of recent history and world events, the average American must rank last among the Group of 20 nations. I believe there are several reasons for this.
The US is vast and the land is so bountiful that America literally needs nothing from overseas. So, why bother with other countries? Additionally, after your basic needs are met the variety of sports/entertainment options available in the US will keep you entertained forever. No time to learn about other countries while the Kardasians are on the tube.