WASHINGTON — The United States imposed 50 percent tariffs on $20 billion worth of Canadian products early Saturday, and Canada immediately said it would retaliate, after last-ditch negotiations failed to resolve the latest strain in already tense relations between the historic allies. President Donald Trump’s import taxes will hit about 5 percent of what Canada ships to the United States every year, including products ranging from hockey sticks to tongue depressors. Canadian Prime Minister Mark Carney said it would match the U.S. tariffs dollar for dollar, escalating the trade conflict and calling into the question the future of a North American trade pact between the United States, Canada and Mexico that is crucial to industry in all three countries. Canada sought concessions on Trump tariffs on steel, aluminum, autos and lumber that the United States was unwilling to provide. “Tonight, Canada declined to finalize the trade deal under the terms agreed earlier this week. Despite the U.S. offer to Canada to receive the best treatment of any major exporter to our market, new demands