New Delhi, Sept. 25 -- UNITED NATIONS - The chairs kept emptying long before Prime Minister Benjamin Netanyahu of Israel reached the lectern. Dozens of delegations rose in near-silent procession and filed out of the General Assembly hall on Thursday, leaving swaths of empty seats behind as Netanyahu prepared to defend a war that a United Nations commission of inquiry has already concluded meets the legal definition of genocide.

The walkout, the largest of this year's General Assembly session, was not spontaneous. Diplomats from Arab, Muslim and African states had signaled for days they would not sit through another Israeli address that skirted the findings of the U.N.'s Independent International Commission of Inquiry on the Occupied Pale...