As Columbia University’s student protest movement careened toward the center of the nation’s political discourse last year, one of its most ardent leaders suddenly fell quiet.

Mohsen Mahdawi had been a key organizer of pro-Palestinian demonstrations, but he said he walked away from that role in March 2024 — well before the rallies reached a fever pitch as students set up encampments and broke into a campus building.

A fissure had been growing. By the fall of 2024 it had widened: Parts of the movement were becoming more radical, and some students were distributing fliers during a campus demonstration glorifying violent resistance. Mr. Mahdawi, meanwhile, was approaching Israeli students, hoping to find middle ground in the divisive Israeli-Palestinian conflict that, for decades, had unleashed horrors on both sides and in his own life.

He told friends that he was being sidelined in part because he wanted to engage in dialogue with supporters of Israel, a stance many pro-Palestinian activists reject.

His calls for compassion did not protect him from President Trump’s widening dragnet against pro-Palestinian student organizers on campus.

At an appointment to obtain U.S. citizenship on Monday in Vermont, Mr. Mahdawi, who is expected to graduate next month from Columbia, was taken into custody by immigration police.