The invasion of Canada by the United States of America seems like something that belongs in fiction, but it's the exact scenario laid out in War Plan Red, an attack strategy drawn up by the US government during the interwar period between WWI and WWII.
If the US carried out War Plan Red, tensions between the two countries would likely be high. The comprehensive strategy — designed as part of an overall war against the United Kingdom — called for an invasion and occupation of Canada to prevent the British from using it as a base of attack. Furthermore, this occupation was intended to be permanent, essentially turning Canada into the 51st state.
The period between WWI and II is mostly known as a time of peace, but the planet's superpowers were still preparing for conflict. The United States of America emerged from WWI as a major victor in terms of combat, economy, and influence, and established its militaristic fortitude. The US government prepared for combat against any nation as the '20s turned into the '30s.
Toward the end of the '20s, they began drawing up a series of color-coded war plans for the nations the US might face off against in potential conflicts. The list was broad and included a number of key allies.
The war plan the United States government and military invested the most interest in was War Plan Red — the official strategy guide for a battle against their former colonial masters in the United Kingdom. The British Empire might not have been what it was before WWI, but the Commonwealth still had a global reach that would play a role in any conflict.
War Plans Red and Crimson — and other stratagems, including Black for Germany, Orange for Japan, and Green for Mexico — weren't just one-off, distant projects. All the color-coded war plans were created by the US Joint Planning Committee — the precursor to the Joint Chiefs of Staff — and they involved input from the Army and Navy at every step of the way.
These decisions occurred at the highest levels of government, as the Secretary of War and the Secretary of Navy needed to approve the plans — which they did in May 1930.
War Plan Crimson was the real meat of War Plan Red. In the event of a conflict with Britain, the United States didn't plan on doing much fighting outside of North America. Instead, the US prioritized an invasion of Canada under the logic that preventing Britain from using their former colony as a base of operations would keep the American homeland safe.
Beyond that, many Americans in power believed they could win a war of attrition with the United Kingdom and saw the occupation of Canada as the key to victory.