CAIRO — After five years of war, the Saudi crown prince, Mohammed bin Salman, appeared to be inching away from his ruinous campaign in Yemen in recent weeks, seizing on the coronavirus pandemic to declare a unilateral cease-fire that, although ineffective, at least signaled that the prince finally agreed with critics who insisted the fight was unwinnable.

His fractious Yemeni allies, however, have other ideas.

A declaration of self-rule over the weekend by Yemen’s leading separatist group, which seized control of the southern port city of Aden and its Central Bank, threatens fresh chaos in the war-torn country.

It comes as the war’s main sponsors, the Saudis and the United Arab Emirates, distracted by their own woes, turn away from the fight. That has left their Yemeni allies, previously united against the Iranian-supported Houthis who control the country’s north, to battle for supremacy.