Tansill was assistant professor of history at the Catholic University of America and
American University. He was full professor of American history at American University from 1921 to 1937. He became professor of history at
Fordham University in 1939, up until 1944. He was Professor of History at
Georgetown University from 1944 to 1957. Over the course of his academic career, he wrote fourteen history books. Tansill published
America Goes to War, a history book about
World War I, in 1938. He finished his review by calling it, "a provocative and authoritative book, which should be on the "must list" of every student of the period." In the 1930s, Tansill was a staunch isolationist, arguing that the United States should not participate in
World War II. During a speech to the
United Daughters of the Confederacy and the
Sons of Confederate Veterans in 1947, Tansill claimed that
Abraham Lincoln was a "do-nothing" soldier and claimed that he had started the
American Civil War by tricking the South into
firing on Fort Sumter. At the same time, he was an advisor to the
United States Senate Committee on Foreign Relations. According to A. S. Winston, Tansill, "blamed
Franklin Roosevelt for forcing a peace-minded Hitler into war and used the standard
Rudolph Hess line that
Hitler wanted only a free hand to deal with
Bolshevism in the East." In an article that he published in
American Opinion, the journal of the
John Birch Society, in 1963, a year before his death, Tansill suggested it would have made sense to impeach President
John F. Kennedy after the latter suggested to the
United Nations that the United States should disarm. ==Personal life==