WASHINGTON — The trade war that erupted Friday between the U.S. and China carries a major risk of escalation that could weaken investment, depress spending, unsettle financial markets and slow the global economy. The opening shots were fired just after midnight, when the Trump administration imposed a 25 percent tariff on $34 billion of imports from China, and Beijing promptly retaliated with duties on an equal amount of American products. It accused the U.S. of igniting “the biggest trade war in economic history.”