The British are coming!

War Plan Red was a contingency plan in the event that the British launched an invasion of the US.  This was a plausible threat in 1930 considering foreign relations at the time and plans where first being drawn up.  By 1934 the scenario was to repel an attack with a poison gas attack on Canada along with fortifying our borders.  By 1935 airfields were being built in the purpose of defense and disguised as civilian airfields.

The declassified document can be found here.

In February 1935, the War Department arranged a Congressional appropriation of $57 million dollars to build three border air bases for the purposes of pre-emptive surprise attacks on Canadian air fields. The base in the Great Lakes region was to be camouflaged as a civilian airport and was to "be capable of dominating the industrial heart of Canada, the Ontario Peninsula" In August 1935, the US held its largest peacetime military manoeuvres in history, with 36,000 troops converging at the Canadian border south of Ottawa, and another 15,000 held in reserve in Pennsylvania. The war game scenario was a US motorized invasion of Canada, with the defending forces initially repulsing the invading Blue forces, but eventually to lose "outnumbered and outgunned" when Blue reinforcements arrive. As for the invasion plan, roads were mapped out and Canadian assets were evaluated. "The best practicable route to Vancouver is via Route 99"

Umm yeah, good luck with that invasion thing.