When the United States entered World War I in 1917, Frankfurter took a special leave from Harvard to serve as special assistant to the Secretary of War Newton D. Baker. He was appointed Judge Advocate General, supervising military courts-martial for the War Department.1https://archive.org/details/peopleshistoryof00iron
In September 1917, he was appointed counsel to a commission, the President’s Mediation Committee, established by President Wilson to resolve major strikes threatening war production. Overall, Frankfurter’s work gave him an opportunity to learn firsthand about labor politics and extremism, including anarchism, communism, and revolutionary socialism. Former President Theodore Roosevelt accused him of being “engaged in excusing men precisely like the Bolsheviki in Russia.”
Frankfurter was encouraged by the Supreme Court Justice Louis Brandeis to become more involved in Zionism. With Brandeis he lobbied President Wilson to support the Balfour Declaration, a British government statement supporting the establishment of a Jewish homeland in Palestine.2https://books.google.com/books?id=T91sokr_nJYC&q=felix+frankfurter&pg=PA112 In 1918, he participated in the founding conference of the American Jewish Congress in Philadelphia, creating a national democratic organization of Jewish leaders from all over the U.S.3https://web.archive.org/web/20071212030021/https://time.com/time/magazine/article/0,9171,788721,00.html In 1919, Frankfurter served as a Zionist delegate to the Paris Peace Conference.