Two nations bound by geography, history, and decades of interwoven commerce now face each other across a newly raised wall of tariffs. After trade negotiations between Washington and Ottawa collapsed this week, the United States imposed a 50 percent duty on $20 billion of Canadian imports — a measure that took effect without delay. Canada has pledged to answer in kind, suggesting that what began as a failure of diplomacy may harden into something neither side fully anticipated. The episode raises an older question: how long can historic allies afford to treat each other as adversaries before t