LONDON — In the more than seven decades since the end of the Second World War, leaders of the planet’s major economies have operated on a rough consensus that trade offers a potent form of inoculation against the outbreak of military hostilities.

The United States and its allies have constructed an economic and security order centered on the notion that communities connected by commerce have a shared interest in maintaining peace. They forged institutions focused on reducing tariffs and other impediments to trade, with the World Trade Organization as the linchpin.

But now this collective understanding is under assault by the very power that has championed it most fervently. As President Trump on Thursday delivered steep tariffs on imported steel and aluminum, provoking vows of retaliation from enraged producers, he risked setting off a global conflict.

Though Mr. Trump stepped back from plans to impose tariffs on every nation — exempting, for the time being, Canada and Mexico — he did so while exerting additional pressure: He emphasized that tariffs could ultimately land on both countries unless they bend to American demands in renegotiations of the North American Free Trade Agreement.