https://fallout.fandom.com/wiki/United_States_annexation_of_Canada
The primary reason for the annexation were Canada's natural resources, considered vital to the American military effort in the Sino-American War.
The tipping point came in 2072. As the U.S. escalated its exploitation of Canada's resources, her people took to the streets. Protests and riots erupted in several Canadian cities and an attempt to sabotage the Alaskan pipeline by unknown forces was foiled, but provided the U.S. Government an excuse to officially annex Canada.
When pictures of the atrocities committed in the name of the United States made it over the border, they sparked unrest and protests from the American public. Both sides thoroughly armed, it had little impact on history, as former Canadian land provided much-needed resources for maintaining the war effort, and the country was overrun.[Non-game 5][Non-game 6]
In a statement to the press, Buzz Babcock, commander of U.S. forces in Canada, cited security concerns as the primary reason for the annexation. He also stated that "Little America" was always the property of the United States and the U.S. military announced the official annexation of Canada on June 3, 2072.
Even among the military, the annexation was met with mixed reactions. For some, the act was considered criminal.[5] Open warfare between the opposing armies across the former nation commenced, with the Yukon becoming one of the many fronts alongside Anchorage in the Resource Wars.[6] Despite the outcry, the military initiative forged ahead. Propaganda portrayed the annexation as a "liberation", with comic series such as Tales from the Front and cartoons such as Armor Ace and the Power Patrol championing U.S. occupation forces while vilifying Canadian resistance; for instance, the villainous "rogue robots" of the Yukon Five in the Armor Ace board game Cold Steel were explicitly described as part of the "Yukon Uprising." Vault-Tec in particular sought to bank on the annexation as they began building and advertising available Vaults in "newly-annexed Canada."
Fallout is supposed to be over-the-top dystopian parody, but here we are.
Just... don't show any of this to the administration, hm?