[citation needed] Immediately after American entry into World War II, Rossiter joined the United States Naval Reserves and served for three years as a gunnery officer, mostly on the USS Alabama in the Pacific Theater, reaching the rank of lieutenant.

[4][5] He spent the 1960–1961 academic year as Pitt Professor of American History and Institutions at Cambridge University, England.

[3] Years after Rossiter's death, his son revealed that his father suffered a lifetime of debilitating clinical depression, which he could no longer extract himself from and overdosed on sleeping pills.

His beloved Cornell University was convulsed with racial conflict, including the occupation of the student union building in April 1969.

[10] In particular, following the events of 9/11, Rossiter's first book, the 1948 Constitutional Dictatorship: Crisis Government in the Modern Democracies (reissued in 1963 with a new preface), was reprinted for the first time in nearly forty years.

His 1964 monograph, Alexander Hamilton and the Constitution, studies the evolution and current relevance of Hamilton's political and constitutional thought, and his 1953 Bancroft Prize-winning Seedtime of the Republic investigates the roots of American thinking about politics and government in the years leading up to the American Revolution.