Quote Originally Posted by Tiberius
Because the Yanks were helping the British, selling them valuable resources such as guns and food. Then, there's the battle of the Atlantic where they are fighting even though techincally not at war, and plenty of American cargo ships get sunk by U-Boots because they've got goods bound for Britain. There's also the factor of supporting Japan against America. The guy was a loon after all, and probably didn't think he would be defeated.
The last sentence you put forward is key. Anybody with real insight would have put up with the undeclared naval war -- Britain lacked the "boots" to do much more without the USA (note: comment on resources, not pluck, the Brits had plenty of that).