"History, proximity, commerce, and shared values underpin the relationship between the United States and Canada. Americans and Canadians fought side by side in both World Wars, Korea, and Afghanistan, and continue to collaborate on international political and security matters, such as the campaign against the Islamic State. The countries also share mutual security commitments under the North Atlantic Treaty Organization (NATO), cooperate on continental defense through the binational North American Aerospace Defense Command (NORAD), maintain a close intelligence partnership through the 'Five Eyes' group of nations, and coordinate frequently on law enforcement efforts, with a particular focus on securing their shared 5,500 mile border. Bilateral economic ties, which were already considerable, have deepened markedly over the past three decades as trade relations have been governed by the 1989 U.S.-Canada Free Trade Agreement and, since 1994, by the North American Free Trade Agreement (NAFTA). Canada is the second largest trading partner of the United States, with total two-way cross-border goods and services trade amounting to over $1.6 billion per day in 2017. The United States is also the largest investor in Canada, while Canada is an important source of foreign direct investment in the United States. The countries have a highly integrated energy market, with Canada being the largest supplier of U.S. energy imports and the largest recipient of U.S. energy exports."
CRS Report for Congress, 96-397