Complex sentence: The 4th regiment had lost considerable strength during the hurried 24 hour march-approximately two-thirds of the soldiers had fallen behind during the march-and only a battalion-sized force had reached the western bank of the Luding Bridge that morning.
Split into simpler sentences: The 4th regiment had lost considerable strength during the hurried 24 hour march. Only a battalion sized force had reached the western bank of the Luding Bridge that morning.