Why would America declare war on the United Kingdom?
It wouldn't. But it is a wise idea for nations to consider and plan for the possibility of war with virtually anyone. The US government used colors to designate these and had at various times in the early 20th century developed plans to fight Japan, Britain/Canada, France, Mexico, Germany, Russia, Internal dissention/revolution, and various combinations thereof.
An all-out War Plan Red war would probably have required PoDs in the late 19th century to make the 20th century virtually unrecognizable to us today.
But while the Anglo-Japanese treaty was still in effect, the US was probably wise to consider the possibility that a War Plan Orange war (with Japan) might ultimately trigger some British involvement on Japan's side. Not because the Treaty obligated Britain to assist Japan in a war with the US (it expressly didn't), but because the planned US strategy of naval blockade and economic starvation of Japan might lead to conflicts with Britain over trade with Japan.
In this instance, the war aims and plans for both the US and Britain would probably be limited, and focused on naval action and very limited air/ground action in peripheral theatres primarily, not an attempt by the US to invade Canada and vise versa. Considering the close relationship that had developed between the US and Britain in the 1900-1919 period, it is virtually inconceivable that relations would have deteriorated to the extent that the US would consider mounting a costly invasion of Canada when the main enemy was Japan. Britain would be even less likely to consider mounting an invasion of the continental US, although I suppose its possible they might attempt to occupy parts of Alaska, or entertain a counter blockade of the US west coast and Hawaii (which was not a major US naval base while the Anglo-Japanese treaty was in effect). I voted "Stalemate" because I thing the war aims would be limited with neither the US or Britain aiming for anything resembling total victory.
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