The shells started landing inside Hussein Ali Wuhaish’s refugee camp days after the Biden administration ended U.S. support for a Saudi-led coalition fighting in Yemen and took steps to lift sanctions on northern rebels.The rebels, known as Houthis, seemingly emboldened by the American pivot, had intensified their offensive in strategic Marib province, one of the few safe harbors in the war. Now Wuhaish and his family, who fled to Marib in 2017, were once again on the run. “I could hear the screams and the crying of my neighbors' children, as my wife and I tried to calm our own children,” said Wuhaish, 38, a doctor and father of five. “We realized that we were again not safe.” So as the sounds of the fighting got closer, we fled for our lives.”The military escalation in Marib, the last northern stronghold of the internationally recognized Yemeni government, threatens to displace hundreds of thousands of Yemenis, many of whom have already fled violence multiple times. That would significantly worsen a humanitarian crisis already described by the United Nations as the world’s most severe, say U.S. and U.N. officials and aid workers. The tensions could implode efforts by the United Nations and others to broker an end to the six-year-long war that has killed or injured tens of thousands in the Arab world’s poorest nation. The conflict pits the Iran-aligned Shiite Muslim Houthis against the U.S.-backed coalition of Sunni Muslim nations, led by Saudi Arabia and the United Arab Emirates, seeking to restore the government to power. Today, the conflict is as much a proxy war between Saudi Arabia and Iran as it is a civil conflict. For the past year, the rebels have steadily advanced in Marib province, roughly 100 miles northeast of Sanaa. Capturing it would remove the government’s last northern footprint and give the Houthis control over lucrative oil and natural gas reserves — a key ambition. The province’s capture would strengthen the rebels’ position in negotiations to end the war and create a new government, analysts say. “Marib was the success story of the war,” said Elisabeth Kendall, a Yemen scholar at Oxford University. “It was a haven in the middle of a war. It’s a place that provided hope. Symbolically, having that completely turn around is really tragic.” If the Houthis seize the province, she said, “it does change the dynamic of the war.”