I will never get why people think a war will start on America's shores... It might start with a bombing here, assuming you can get past the RADAR, but there's not a chance in hell that a sizable invasion fleet could reach here unless they came from Canada/South America and we somehow didn't notice. The U.S. Navy is just too big and the RADAR/Satellite system too thorough for it to happen. My bet, if one does actually happen, it will involve China, since they are the main power in Asia for the next 30-50 years. It is more likely to start with India/Pakistan or in North/South Korea (don't forget, they've been at war for 60 years now). I could see it easily involving all of Asia, and through treaties and various bases, America and likely Britain.
For those saying an Asian war wouldn't technically be a World War, I would like to remind them that WWI was largely fought in Europe, with a smaller campaign in the Middle East/North Africa. WWII was the only war that ever truly spanned the globe.
As for the person saying France and Britain started WWII, no. They issued the ultimatum that brought them into the war, but Germany had just invaded Poland.