Complex sentence: In March 1942 Hawkesworth was appointed General Officer Commanding (GOC) of the 4th Infantry Division shortly after which a tank brigade (the 21st) was substituted for one of the three infantry brigades (the 11th) to create a "mixed" division.
Split into simpler sentences: In March 1942 Hawkesworth was appointed as General Officer Commanding (GOC) of the 4th Infantry Division. Shortly after he assumed command a tank brigade (the 21st) was substituted for one of the three infantry brigades (the 11th) to create a "mixed" division.