The United States and Canada have wrangled for decades over trade, poking each other interminably over sore spots like Canadian softwood lumber imports and U.S. access to Canada's protected dairy market.

Somehow the two neighbors still managed to remain friends, allies — and trading partners. Canadian soldiers fought alongside Americans in Afghanistan after 9/11. The 5,525-mile U.S.-Canada border is undefended, and nearly 330,000 people and $2 billion dollars' worth of goods cross it every day; 800,000 Canadians live in the United States.