They eventually came in in 1917, but they were reluctunt. The same with 2nd world war, as you saythey again only came in because of pearl harbour. But a lot of the american help cost a lot of money, it wasnt free. Example Britain only settled up its bill with america last yea. What annoys me is half the equipment britain bought off them was shit. But your right america will only go to war if theres something in it for them. Iraqs oil been an example of this.