Forgot to mention: If they wall off Canada, better hope the Russians don't get frisky. After all, the Pine Tree Line won't give enough warning to do anything. Note that there are 2 other lines further out than the PTL (DEW line, and Mid-Canada line), but as the name for one of them implies, they're on Canadian soil, so if the wall goes up U.S. personnel will be "invited" to leave the facilities.
Also, trace out the "great circle" routes from Europe to North America - U.S. based airlines would be at a competitive disadvantage if they had to stay out of Canadian airspace (and service from Washington to Alaska would be crippled). Sure Canada would have longer routes for transcontinental flights, but it wouldn't be nearly as bad as what the U.S. would face.
Also, trace out the "great circle" routes from Europe to North America - U.S. based airlines would be at a competitive disadvantage if they had to stay out of Canadian airspace (and service from Washington to Alaska would be crippled). Sure Canada would have longer routes for transcontinental flights, but it wouldn't be nearly as bad as what the U.S. would face.
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