On his return to America, Flood turned to America’s revolution. Rise and Fight Again won the New York American Revolution’s Round Table’s Annual Award in 1976, the nation’s bicentennial year. Next Flood turned to the war that disrupted the Union. Lee – The Last Years was a revelatory look at how Robert E. Lee reacted to the South’s defeat. It won the Colonial Dames of America Annual Book Award.
Lee was followed by Grant and Sherman –the Friendship that Won the Civil War ; 1864: Lincoln at the Gates of History; and Grant's Final Victory – the story of the last anguished year of Grant’s life, during which, while dying of cancer, he struggled to finish his memoirs and save his family from penury.
Flood completed The Lafayette Escadrille – the story of the Americans who volunteered to serve as pilots on the Western Front in 1915 before the United States entered World War I. Historian Thomas Fleming has praised it as a rare combination of heart-wrenching idealism and breathtaking suspense. The book was published in 2015, after his death.
Over the years, Flood published many short pieces in The New Yorker, The Atlantic, Esquire, and other magazines. He was also an active member of the literary community. He was president of the American Center of PEN, the international writers’ organization, and served on the governing bodies of the Authors League and Authors Guild. He travelled widely across Europe, Asia, and Africa; his time in Africa included a successful summit of Mt. Kilimanjaro and served as the inspiration for his novel Tell Me, Stranger. In 1970, Flood met Katherine Phelps Burnam of Richmond, KY who was then working in Manhattan. They married and moved to Richmond in 1975 and lived on a farm there until his death. He spent his summers in the coastal town of Brooksville, ME.