London, October 15, 1944

The Marquess of Hartington spent only five weeks with his bride before being sent back to the war where he died in action less than three months later, on August 12, 1944. Kathleen herself would die in a plane crash only four years later, at 28 years old. After two years of widowhood, in 1946, she began a relationship with Earl Fitzwilliam, of whom her parents disapproved. In 1948, after learning her father, Joseph P. Kennedy, Sr., was to be in Paris, and wanting to try and gain his consent to marry, Kathleen and Fitzwilliam chartered a plane, leaving May 13. 

From Wikipedia:

At 3:30 in the afternoon, their plane took off, reaching an altitude of 10,000 feet. Approximately one hour into the flight, radio contact was lost with the plane when it entered the region near Vienne which was also close to the center of a storm. The plane’s four occupants endured twenty minutes of severe turbulence which bounced their small plane up and down as much as several thousand feet at a time.

When they finally cleared the clouds, they instantly discovered the plane was in a dive and moments away from impact, and they attempted to pull up. The stress of the turbulence coupled with the sudden change of direction tore loose one of the wings, followed by both engines and finally the tail. The plane’s fuselage then spun into the ground seconds later, coming to rest nose down in a ravine after striking terrain near the summit of Le Coran, the highest of the Cevennes Mountains in the Saint-Bauzile, Ardèche, France. Kennedy was instantly killed along with Fitzwilliam, pilot Peter Townshend, and navigator Arthur Freeman.