Many flights from Western Canada to southern Ontario pass over US territory. The US is now making noises that they may require passengers on such flights to be pre-approved by US authorities up to 72 hours in advance. This also applies to Canadian flights to Mexico, Cuba and other points south.
It will be politically impossible for the Canadian government not to impose similar conditions on US flights passing over Canada. This included flights from Alaska to the Eastern US, and most flights from the US to Europe.
It'll be amusing to watch the reaction of some US congressman travelling from Washington to Alaska or Europe, when Canada denies him permission to get on the flight because he didn't book far enough in advance.
I guess the flights to Seattle could veer out over the ocean a bit to avoid this, but will they?
And another reminder that what we do without thinking has long term consequences:
China raised its visa fees for Americans to match what the US charges foreigners. They also require you to come to the embassy or consulate in person as the US does in China. Fortunately, the Chinese did this only symbolically - you can hire someone to take your passport in for you and get a visa if you don't have a local consulate.
Brazil also instituted fingerprinting of Americans coming into Brazil in retaliation for this requirement by American Customs.
Let's hope wiser heads prevail.