Today, Canada and the United Kingdom are among America's closest friends and trading partners. This, though, was largely a product of World War II. For the bulk of the 19th and early 20th centuries, the relationship between the US and the Commonwealth of Nations was chilly at best. That includes the period following the Great War.
Yes, even after America, Canada, and the UK teamed up to help defeat the Central Powers, there was a very real possibility that we'd see an open Anglo-American conflict, with Canada being the likely battleground. As The National Interest tells it, the Washington Naval Treaty and the overall exhaustion that lingered after World War I prevented such a conflict. The site did perform a detailed analysis of the what-if, though. How would the chips fall if Uncle Sam invaded Canada? How would the British, the Canadians, and even the Quebecois and Japanese respond?
It makes for truly delicious reading, especially for fans of alternate history (if you've watched Amazon's series The Man in the High Castle, or the excellent book it's based on, you'll love this). As NI contributor Robert Farley tells it, the linchpin of a US invasion would have been the capture of Halifax, which would effectively cut off Canada from British support. From there, it'd be a matter of not if but when cities like Toronto and Ottawa would surrender. At the same time, the US Navy's focus on an Atlantic fleet – there's a great breakdown of the British and US naval forces – and the close relationship between the UK and Japan would likely see an invasion of the America's Pacific possessions.
Head over to The National Interest to read up on this interesting hypothetical.