For four years, the United Arab Emirates have been the military linchpin of the Saudi-led war in Yemen, providing weapons, money and thousands of ground troops to a campaign to drive out Yemen’s Houthi rebels. Emirati forces led almost every major advance the coalition made.

Now they have decided they can go no further.

The Emiratis are withdrawing their forces at a scale and speed that all but rules out further ground advances, a belated recognition that a grinding war that has killed thousands of civilians and turned Yemen into a humanitarian disaster is no longer winnable.

Emirati officials have been saying for several weeks that they have begun a phased and partial withdrawal of forces estimated at 5,000 troops a few years ago.

But Western and Arab diplomats briefed on the drawdown say a significant reduction has already occurred, and that the Emiratis are driven mostly by their desire to exit a war whose cost has become too high, even if it means angering their Saudi allies.