Year after year, the bombs fell - on wedding tents, funeral halls, fishing boats and a school bus, killing thousands of civilians and helping turn Yemen into the world's worst humanitarian crisis. Weapons supplied by American companies, approved by American officials, allowed Saudi Arabia to pursue the reckless campaign. But in June 2017, an influential Republican senator decided to cut them off, by withholding approval for new sales. It was a moment that might have stopped the slaughter. Not under President Trump. Trade adviser Peter Navarro ... wrote a memo to Jared Kushner and other top White House officials calling for an intervention. He titled it Trump Mideast arms sales deal in extreme jeopardy, job losses imminent." Within weeks, the Saudis were once again free to buy American weapons. The intervention, which has not been previously reported, underscores a fundamental change in American foreign policy under Mr. Trump. Where foreign arms sales in the past were mostly offered and withheld to achieve diplomatic goals, the Trump administration pursues them mainly for the profits they generate and the jobs they create, with little regard for how the weapons are used. Mr. Trump has tapped Mr. Navarro ... to be a conduit between the Oval Office and defense firms. His administration has also rewritten the rules for arms exports, speeding weapon sales to foreign militaries. The State Department, responsible for licensing arms deals, now is charged with more aggressively promoting them.