The United States-Mexico-Canada Agreement, which replaced the North American Free Trade Agreement (NAFTA), binds almost half a billion consumers in a single market.
The very fact that Keystone XL isn't uniquely unsafe nor unpopular - despite loud activist opposition, the people who actually live near its proposed path vote overwhelmingly for project supporters, including all six U.S. Senators in the three most impacted states - suggests that no oil or natural gas-related project is safe from White House opposition.
Mr Trudeau himself raised the pipeline as a top priority in a call with Mr Biden in November, trying to make the case for going forward with the project. Alberta's Jason Kenney, Ontario's Doug Ford, Saskatchewan's Scott Moe and Quebec's François Legault all pushed the prime minister to take action to save Keystone.
Kenney told MacCallum Alberta ships $100 billion worth of energy to the US annually and that KXL would've increased that shipment in "safe, modern" way.
"The United States is setting a deeply disturbing precedent for any future projects and collaboration between our two nations", Kenney wrote in a letter to Trudeau he released Friday on Twitter. His executive order, which fulfilled a campaign promise, cited concerns about climate change.
The White House statement said Biden acknowledged Trudeau's disappointment.
"Most major oil and gas companies have already opted out of oilsands projects not only due to their high breakeven oil price, but also because of its association with dirty and non-sustainable oil, since traditional oilsands require mining and consume high volumes of water".
The pipeline is an addition to an existing pipeline system transporting hundreds of thousands of barrels of crude oil from Canadian tar sands to American refineries. "Keystone XL would have meant a significant, safe, modern increase in that shipment", Kenney said. They say this could help "invest in building offshore renewable energy and put Americans to work doing it".
He called the decision a violation of the investor-protection provisions of the U.S. -Mexico-Canada Agreement and called on Trudeau to press the U.S. for compensation on behalf of TC Energy and the Alberta government.
"I think that this is kind of a new moment and full of new opportunities for a very strong good relationship", the second official said, describing the call as "genuinely warm and collaborative".