The United States formally entered World War II in December 1941, following Japan's surprise attack on Pearl Harbor in Hawaii. Six months later, Bush enlisted into the U.S. Navy immediately after he graduated from Phillips Academy on his eighteenth birthday. He became a naval aviator, taking training for aircraft carrier operations aboard USS Sable. After completing the 10-month course, he was commissioned as an ensign in the United States Naval Reserve at Naval Air Station Corpus Christi on June 9, 1943 (just three days before his 19th birthday), which made him one of the youngest aviators in the U.S. Navy.
In September 1943, he was assigned to Torpedo Squadron 51 (VT-51) as the photographic officer. The following year, his squadron was based in USS San Jacinto as a member of Air Group 51, where his lanky physique earned him the nickname "Skin". During this time, the task force was victorious in one of the largest air battles of World War II: the Battle of the Philippine Sea.
Read more here on wikipedia.
