really? which year? Face it, all of the English speaking nations essentially demobilized their armies post 1919. The RN and USN were maintained at a significant size, but the great armies of 1918 are long gone, The British are in financial trouble, the US has no interests that clash with the British Empire and what exactly is the scenario that leads to any kind of significant friction, much less war?
While the Canadian Corps has a lot of battle honors and much to be proud of, it no longer exists post war, no do the massive American divisions of that era either.
So what year are you making assumptions about? The British is doing good to put together a division for maneuvers by the mid 20s, and while still better off than the US Army, none of the English speaking nations can be remotely be described as combat ready in any significant sense.
The only possibility of any kind of armed clash is pre World War I, and even then as we get closer to 1914 and it becomes increasingly clear that Germany and Britain are going to clash any friction between the US and Britain is pretty much in the dustbin of history.
Contrary to what you might think, aside from a few wackos, no one in the United States has had dreams of conquering Canada since the mid 19th Century.