Israeli tanks seized control of Gaza’s vital Rafah border crossing on Tuesday as Israel brushed off urgent warnings from close allies and moved into the southern city even as cease-fire negotiations with Hamas remained on a knife’s edge. The U.N. warned of a potential collapse of the flow of aid to Palestinians from the closure of Rafah and the other main crossing into Gaza, Kerem Shalom, at a time when officials say the northern part of the enclave is already experiencing “...