Complex sentence: The bulk of Cao Cao's troops had already crossed the river, leaving Cao and his Tiger Warriors to bring up the rear, when Ma Chao and his 10,000 horsemen caught up with them.
Split into simpler sentences: The bulk of Cao Cao's troops had already crossed the river, leaving Cao and his Tiger Warriors to bring up the rear. Just then, Ma Chao and his 10,000 horsemen caught up with them.