Fortress North America is a term used both during the Second World War and more often in the Cold War to refer to the option of defending Canada and the United States against their enemies if the rest of the world were lost to them.
It was viewed only as a last-ditch option in case Europe and Asia were overrun by the fascist or Communist. At the outset of the Cold War there were some, especially in the United States, who supported the isolationist idea of fortifying North America and abandoning international involvements. This option was rejected with the formation of NATO and the decision to permanently station troops in Europe.
During the Cold War significant planning and effort went into developing continental defense systems just in case. Most notable were the formation of NORAD and the setting up of radar lines in the Canadian Arctic. Canadians were long concerned that the adoption of a Fortress North America strategy involving close intergovernmental links and the loss of outside links would inevitably result in the nation's absorption by the United States.
In the aftermath of the September 11th attacks the idea of Fortress North America has been revived as a strategy of keeping both nations safe from terrorism while keeping the Canada/U.S. border undefended and open to trade.