Coster served in the Navy from 1941-45, most of that time in the Pacific, including as chief gunnery officer on a troop transport that landed the first marines at Guadalcanal and narrowly escaped destruction in the Battle of Savo Island, a major defeat for the U.S. Navy. He was at sea for weeks and months at a time, with no mail service. When he could send letters, he used guarded language so the censors would let them through.
After the end of the war, Coster took a job in Washington that he describes in vague terms in his letters. That job took him to Shanghai, where he observed the civil war that raged between the Nationalists and the Communists. He is careful in his letters to avoid specifics, but it is likely that he was sent to China in an intelligence role, probably by the Office of Strategic Services (OSS), the precursor to the CIA.
[Coster wartime letters]