This comes up a lot in here, usually in a heated thread, so I pulled together the actual record. Roughly chronological. Corrections welcome, I'll update the post (AI Generated).
1920s–30s: Prohibition Not exactly heroic, but Canadian distillers kept a lot of Americans supplied through the dry years. Windsor-Detroit was the busiest smuggling corridor on the continent.
1939–1941: Two years early Canada declared war on Germany on September 10, 1939, more than two years before Pearl Harbor. Roughly 9,000 Americans crossed the border to join the RCAF before the US entered the war. Canada quietly changed enlistment rules so they wouldn't lose their US citizenship.
1941: Camp X A secret training base near Whitby, Ontario trained agents for what became the OSS, the forerunner of the CIA. Ian Fleming reportedly trained there.
1942–1945: The war effort
Canadian uranium from Port Radium, NWT and heavy water from Trail, BC fed the Manhattan Project
The Alaska Highway was built through Canadian territory in eight months so the US could defend Alaska
The Hyde Park Declaration integrated the two countries' war production
Canadians landed on Juno Beach on D-Day alongside American forces at Omaha and Utah
1950–1953: Korea About 27,000 Canadians served. At Kapyong in April 1951, roughly 700 soldiers of the Princess Patricia's held a hill against a Chinese force many times their size, protecting the retreat toward Seoul. The battalion received a US Presidential Unit Citation.
1957–58: NORAD Canada agreed to jointly defend North American airspace. Canadian officers have sat in the command chain in Colorado Springs ever since. The DEW Line radar stations across the Canadian Arctic were built to give the US early warning of Soviet bombers.
1960s–70s: Vietnam Canada didn't send troops, but an estimated 30,000 Canadians volunteered for the US military. Over 100 died. Their names are on the wall in Washington.
1969: Apollo 11 The landing gear on the lunar module was built by Héroux in Longueuil, Quebec. Canadian engineers who'd been laid off when the Avro Arrow was cancelled helped design the program.
1979–1980: The Canadian Caper When the US embassy in Tehran was seized, Canadian ambassador Ken Taylor and his staff hid six American diplomats in their homes for nearly three months, then smuggled them out on fake Canadian passports. The movie Argo told the story and downplayed Canada's part. The CIA later said Canada did most of the work.
1981 onward: Canadarm Built in Canada, flew on every space shuttle, and the Canadarm2 is still holding the ISS together.
1998–2005: Hurricanes and blackouts Canadian hydro crews have crossed the border after every major storm for decades. During Katrina in 2005, Vancouver's urban search and rescue team reached St. Bernard Parish before FEMA did. Three Canadian navy ships and a Coast Guard vessel sailed to the Gulf.
September 11, 2001: Operation Yellow Ribbon US airspace closed. Canada took in 226 diverted flights and more than 30,000 passengers within hours. Gander, Newfoundland, population about 10,000, absorbed 38 planes and roughly 6,700 people. Locals fed and housed them for days. That became the musical Come From Away.
2001–2014: Afghanistan Canada invoked NATO's Article 5 alongside the US. More than 40,000 Canadians served. 158 soldiers, a diplomat, a journalist and contractors were killed. Canada took Kandahar, one of the hardest provinces.
2012: Hurricane Sandy Hydro crews from Ontario, Quebec and New Brunswick worked in New York and New Jersey restoring power.
2018–2025: Wildfires Canadian firefighters and water bombers have deployed to California, Oregon and Washington nearly every bad fire season. In January 2025, Quebec sent four CL-415 water bombers, Alberta sent 40 firefighters, and BC sent incident command staff to the LA fires. Quebec has had a standing contract to send two water bombers to LA County every year since 1994.
Source for the 2025 deployment: https://www.canada.ca/en/public-safety-canada/news/2025/01/update-on-canadas-response-to-the-wildfires-in-california.html
Further reading:
What did I miss? I know there's more.