By the summer of 1940, Jack was a newly-minted graduate of Harvard and also an accomplished author. Yet he still had no idea what he was going to do with the rest of his life. In the meantime, he traveled to the West Coast and enrolled at Stanford University.

Jack was never a serious student at Stanford. Instead, he mostly enjoyed the social life Stanford University had to offer, especially the University's one great advantage over Harvard: it was co-ed. The only course he attended was Professor Theodore Krep's, " Introduction to Business and Government" .  Jack, however, showed no great enthusiasm for the course. 

On October 29th 1940, as the result of a highly-publicized lottery, the initial group of young men was drafted into Americas€™ first peacetime conscript army. Jack was the first Stanford student to be drafted. The draft results appeared on the front page of the Stanford student newspaper. Jack's service was deferred until the end of the year so he could finish his studies.

One of Jack's most important actions that year was to convince his father to moderate his views opposing aid to Britain. Here, Jack's efforts seem to have been a success. As a result, his father gave a much-ballyhooed speech, in which he announced his support for Lend-Lease. In January 1941, Jack was hospitalized once again. From his bed, he wrote an article urging Ireland to grant Great Britain request for military bases on Irish soil. 

In the spring of 1941, Jack traveled to South America. On his return, he decided to enlist. He was turned down by the Army, and once by the Navy. Jack was able to seek the assistance of the former Naval Attaché in London, now the Director of Naval Intelligence. Captain Alan Kirk facilitated Jack's entrance into the Navy. Jack applied for Naval Intelligence. After being interviewed, a summary was written by Commander E. M. Major which stated: "Subject's education, ability, unusually wide acquaintance and background, together with his personal qualifications fit him for commission as Ensign, IV(S). Subject appears qualified for service in OP-16F in the Office of Naval Intelligence. "

On October 17th, 1941, Ensign Kennedy was called to active duty in the Foreign Intelligence Branch of the Division of Naval Intelligence. Back at home, the first of what were to be many, Kennedy family tragedies occurred. Rosemary, the eldest daughter of the Kennedy clan suffered from mental illness. As she got older, Rosemary became more difficult to handle. Doctors recommended radical surgery. The Kennedys agreed. The surgery, which turned out to be a frontal lobotomy, left Rosemary severely impaired. She was to be institutionalized for the rest of her life.

Jack easily settled into Washington DC naval life. He had a 9-to-5 job. In the evening, he was free to participate in the Capital's social life. His sister Kathleen was working in Washington as a journalist. Together, Jack and Kathleen traveled in the most rarified social circles of Washington. 

It was through his sister Kathleen that Jack met Inga Arvard. A married Danish journalist, Arvard had met with Goering and Hitler. Like Jack, Arvard was 28 years old. She was also intelligent, beautiful, worldly and sexy. They met when she was sent to interview him for The Times Herald. Her interview concluded that, " The 24 years of Jack's existence on our planet have proved that there is really a boy with a future. " Inga and Jack began an intense love affair, an affair that was not affected by the fact that she was suspected of being a Nazi spy. Jack was told to end the relationship by members of his family as well as his Navy superiors. The FBI taped some of his meetings with Arvard. In addition, there was talk of the threat of a LIFE Magazine story about their affair. 

Jack was eventually assigned to work in Charleston with defense contractors. The combined pressure from all sides ended the love affair. Inga broke it off, just before Jack headed into the combat Navy. Ensign Kennedy was ordered to Northwestern University to learn the art of being a sea-going Naval Officer. He completed the course in 60 days. While at Northwestern, Jack became intrigued by the tales of the PT (patrol boats) in the Pacific. He volunteered for PT service. After completing his course, Jack was sent to the new PT school at Melville, Rhode Island. Upon his arrival, he was promoted to Lieutenant (j. g.). and went through an additional eight-week training course. 

At the end of the course, Kennedy was assigned to be an instructor. It appears that his father had used some insider connections to keep his son in the US. Jack resented his father's involvement and used connections of his own (probably through his grandfather) to reach the Chairman of the Senate Naval Subcommittee who managed to get Jack's orders changed. He was then ordered to take a PT boat to Florida, after which he was assigned to a squadron of PT boats sent to guard the Panama Canal. This was not what Lieutenant Kennedy had had in mind. Using his connections once more, he had new orders issued directing him to the South Pacific. 

Kennedy was transported to the Pacific on board the transport ship, Rochambeau. Almost upon his arrival in the Solomon Islands, he saw the captain of his ship being killed in a Japanese air raid. At Esprito Santo, he was sent on to Sesapi, the headquarters for PT boats. On April 23rd, 1943, Kennedy took command of PT€“109. This wooden boat was powered by turbine engines using highly flammable aviation fuel. PT€“109 had already seen its share of combat. It was Kennedy's job to put the boat back in order and to assemble a new crew.

By the summer of 1943, PT€“109 was ready for action. Its job, as well as the job of the rest of the squadron, was to intercept the " Tokyo Express" , the ship bringing supplies to Japanese troops. On the night of August 1st, 1943, 14 PT boats were ordered into the Blackett Straits. In the midst of the patrol, on a dark night, the lookout on PT€“109 suddenly spotted a large shape looming down on them. It was the Japanese destroyer " Amigari" which hit the PT boat and split the boat. Members of the crew landed in the water and were badly injured. Kennedy swam heroically that night, rescuing crewmen.

The boat stayed afloat overnight, but was taking on water. By the next afternoon, it was clear the boat was going to sink. The crew swam to a nearby island, with Kennedy towing the most seriously- injured crew member the entire distance. 

Once on the island, Kennedy swam out to find a passing PT boat, but failed. On the night of the third day, Kennedy met some natives. Upon learning that the crew members were not Japanese, the natives proved willing to help. They carried a message to an Australian coast watcher, who conveyed word to the US Navy. Kennedy, and all but two crew members (who were lost the first night) were rescued. Jack's heroic rescue efforts were big news. The New York Times headlined: " Kennedy's son A Hero in the Pacific as Destroyer Splits" . It was a great story, and the fact that Kennedy came from a prominent family only added to its luster.  Jack Kennedy had become an instant national hero.