President Trump has opened a rift with one of America's closest allies. But things have been worse. Here's everything you need to know: Are the two countries close? The strong, neighborly ties between Canada and the U.S. are virtually unprecedented in world history. The two nations share the world's longest border, which is mostly undefended except for civilian law enforcement, and the American and Canadian militaries work hand in hand for joint defense through the North American Aerospace Defense Command, or NORAD.