On July 14, 1918, Quentin Roosevelt, a pilot in the United States Air Service and the fourth son of former U.S. President Theodore Roosevelt, is shot down and killed by a German Fokker plane over the Marne River in France.

The young Roosevelt was engaged to Flora Payne Whitney, the granddaughter of Cornelius Vanderbilt, one of the country’s richest men. The couple met at a ball in Newport, Rhode Island, in August 1916 and soon fell in love, although the alliance between the modest, old-money Roosevelts and the flamboyantly wealthy Vanderbilt-Whitneys was at first controversial on both sides.

Quentin’s letters to Flora, from the time they met until his death, charted the course of America’s entry into the war. Theodore Roosevelt, incensed at America’s continuing neutrality in the face of German aggression–including the sinking of the British cruise liner Lusitania in May 1915, in which 128 Americans drowned–campaigned unsuccessfully for the presidency in 1916, severely criticizing Woodrow Wilson, who was reelected on a neutrality platform. While he was initially neutral, Quentin came to agree with his father, writing to Flora in early 1917 from Harvard University, where he was studying, that “We are a pretty sordid lot, aren’t we, to want to sit looking on while England and France fight our battles and pan gold into our pockets.”

After U.S. policy, as well as public opinion, shifted decisively towards entrance into the conflict against Germany, Wilson delivered his war message to Congress on April 2, 1917. At age 20, Quentin was too young to be drafted under the subsequent military conscription act, but as the son of Theodore Roosevelt, he was certainly expected to volunteer. His father, at 58, had expressed his own intention to head to France immediately as head of a volunteer division; upon Wilson’s rejection of the idea, TR declared that his sons would go in his place.

Before the month of April 1917 was out, Quentin had left Harvard, volunteered for the U.S. Air Service and proposed to Flora. The young couple received their parents’ consent, at first reluctant, only to say goodbye to each other at the Hudson River Pier on July 23 as Quentin set sail to France for training. Over the next year, Quentin struggled with difficult flight training (on Nieuport planes, already discarded by the French as a second-rate aircraft), brutally cold conditions, illness (in November he caught pneumonia and was sent to Paris on a three-week leave) and derision from his older brothers, Ted, Archie and Kermit, all of whom were already on their way to the front. Quentin also suffered from the separation from Flora, whom he urged to find a way to come to Paris and marry him; though she tried, she was ultimately unsuccessful. Despite the pain of separation from his beloved, Quentin was determined to get to the front, to silence his brothers’ criticism and prove himself to them and to his father.

In June 1918, Quentin got his wish when he was made a flight commander in the 95th Aero Squadron, in action near the Aisne River. “I think I got my first Boche,” he wrote in excitement to Flora on July 11, referring to a German plane he had shot at during a flight mission. Three days later, during the Second Battle of the Marne, his Nieuport was engaged by three Boche planes, according to one of the other pilots on his flight mission. Shot down, Quentin’s plane fell behind the German lines, near the village of Chamery, France.

Flora Payne Whitney saved every one of Quentin’s letters to her. She became a surrogate member of the Roosevelt family for a time, nursing her own pain and comforting Theodore Roosevelt, who was by many reports shattered by the loss of his youngest son, until his death in January 1919. She would later go on to marry twice, have four children, and follow her mother, the sculptor and art patron Gertrude Vanderbilt Whitney, into a leadership role at the Whitney Museum of American Art in New York City. She died in 1986.