Mark Carney swept to power on a backlash to President Trump’s talk of making Canada the 51st state, which many Americans took for mere shtick. But for the new prime minister, reading intelligence reports detailing the gravity of the crisis, it was a breaking point.  In private phone conversations with Carney’s predecessor, Justin Trudeau, Trump had threatened to scrap the 1908 agreement delineating their shared border. “I tear that up and your whole country unravels,” Trump told Trudeau in one call, according to two people familiar with the matter. Over dinner at Mar-a-Lago, Trudeau’s envoys tried to dissuade Trump from…