When a civil war in Yemen erupted more than three years ago, it fractured what was already the poorest Arab nation and eventually plunged it into the planet’s worst humanitarian disaster.

While the world has turned its gaze from the prolonged conflict, it has ground on without easing and grown increasingly complex.

An assault by the Saudi-led coalition that has been battling Iran-backed Yemeni rebels for more than three years began Wednesday on the city of Al Hudaydah, which has a port that serves as a vital route for humanitarian aid and other vital supplies to the bulk of Yemen’s population. It was the latest turn in a situation moving toward catastrophe for millions of civilians.