The American-backed government of Yemen abruptly collapsed Thursday night, leaving the country leaderless as it is convulsed by an increasingly powerful force of pro-Iran rebels and a resurgent Qaeda. The resignation of the president, prime minister and cabinet took American officials by surprise and heightened the risks that Yemen, the Arab world’s poorest country, would become even more of a breeding ground for Al Qaeda in the Arabian Peninsula, which has claimed responsibility for audacious anti-Western attacks — including the deadly assault on Charlie Hebdo in Paris this month. The resignation of President Abdu Rabbu Mansour Hadi brought full circle Yemen’s Arab Spring revolution, which ousted former President Ali Abdullah Saleh in 2011 amid massive popular protests. Now Mr. Saleh, who has lately made himself an unlikely ally of the Houthi rebels who toppled the government, is poised to return to the forefront of Yemeni politics. […]