How did I come up with those dates? Simple, China would need to build up a Navy and Russia would need to rebuild and refurbish their existing Navy, and Britain, well they can not dominate the world like they used to because they lost their colonies, so I just went back to when they were at the height of their power IMO, if they truly tried to retake America they most likely would have succeeded.
BTW, can anybody tell me the strategic importance of capturing those lands in gray?
http://img158.exs.cx/img158/8733/usoutline1cl.jpg