Jackson joined the Army when the United States entered World War II but left when Franklin D. Roosevelt ordered all representatives to return home or resign their seats. He visited the Buchenwald concentration camp a few days after its liberation in 1945. He attended the International Maritime Conference in Copenhagen, Denmark, in 1945 with the American delegation, and he was elected president of the same conference in 1946, when it was held in Seattle. From 1945 to 1947, Jackson was also the chairman of the Committee on Indian Affairs.