Conversation so far:
What happened at Tarawa with David M. Shoup?
In mid-1943, Shoup was transferred to the staff of Major General Julian C. Smith, commander of the 2nd Marine Division, and planned the invasion of Betio on Tarawa Atoll.
Was David M. Shoup's invasion of Betio successful?
David M. Shoup's Battle of Tarawa resulted in an American victory.
What happened after the invasion of Betio with David M. Shoup?
For his leadership during the assault and the push inland David M. Shoup was awarded the Medal of Honor and the British Distinguished Service Order.
Did David M. Shoup remain in service or did he retire after the Battle of Tarawa?
In December 1943, Shoup became chief of staff of the 2nd Marine Division, which was then refitting and training in Hawaii for the upcoming invasion of the Marianas in June.

Question: Why was he relieved that night instead of waiting?
Self-contained rewrite: Why was David M. Shoup relieved by Colonel Merritt A. Edson instead of waiting at the Battle of Tarawa?