Race to replace Prime Minister Justin Trudeau’s government is suddenly competitive after U.S. president calls for tariffs and says Canada should be the 51st state.

Should Canada take U.S. President Donald Trump’s annexation threats more seriously? Military historian Marc Milner says yes, but an invasion from the south — however unlikely — would spell disaster for Canada.

The four contenders to become Canada's next prime minister debated twice this week—and President Trump, who has threatened to make Canada the 51st state, was a main topic of conversation in both debates.