WASHINGTON — Determined to wall off America's border with Mexico, President Donald Trump triggered a diplomatic clash and a fresh fight over trade Thursday as the White House proposed a 20 percent tax on imports from the key U.S. ally and Mexican President Enrique Pena Nieto abruptly scrapped next week's trip to Washington. The swift fallout signaled a remarkable souring of relations between Washington and one of its most important international partners just days into the new administration. The U.S. and Mexico conduct some $1.6 billion a day in cross-border trade, and cooperate on everything from migration to anti-drug enforcement to major environmental issues.