Complex sentence: Concern at General Headquarters about the state of the Cavalry Corps to the north, led to III Corps being ordered to provide reinforcements and two infantry companies and part of an engineer company were sent to Messines in the afternoon.
Split into simpler sentences: Concern at General Headquarters about the state of the Cavalry Corps to the north, led to III Corps being ordered to provide reinforcements. Two infantry companies and part of an engineer company were sent to Messines in the afternoon.