In the second half of his House career, Fish was a chief critic and opponent of President Franklin D. Roosevelt, especially on matters of international affairs and American entry into World War II prior to the attack on Pearl Harbor.

He served three terms before enlisting in World War I, in which he commanded a company of the 369th Infantry Regiment, a unit of African-American soldiers known as the "Harlem Hellfighters."

Throughout the 1930s, Fish was the subject of multiple foreign influence campaigns, since he was identified by Nazi Party officials as a natural ally to their international ambitions (though he was on the record criticizing the treatment of Jews in Germany) and by British security organizations as an obstacle to American aid for Great Britain.

In 1941, Fish was implicated in an America First Committee franking controversy leading to William Power Maloney's grand jury investigating Nazi penetration in the United States.

[1] After the 1941 Japanese attack on Pearl Harbor and German declaration of war, Fish called for unified support for Roosevelt as a wartime president.

Late in the decade, former United States Department of Justice prosecutor O. John Rogge accused Fish of Nazi sympathies.

[9] Yale ultimately withdrew from the Assembly race in favor of a political ally, whom Fish easily defeated running on an anti-corruption platform.

[9] With support from Theodore Roosevelt, Fish opposed a bill which would allow insurance companies to bypass the workmen's compensation scheme to settle claims and attacked the Speaker of the Assembly, Thaddeus Sweet, for misuse of public funds and hiring political allies for no-show jobs.

The summer after President Wilson's declaration of war against Germany (in April 1917), Fish and about two thousand soldiers began training at Camp Whitman (in New York).

In November 1917, the regiment boarded the USS Pocahontas, destined for France, although shortly thereafter the ship returned to shore due to engine problems.

[13] In addition, Fish and his sister Janet, who had been a nurse near the front lines, were both later inducted into the French Legion of Honor for their wartime service.

"[25] Finally, in part under the influence of New York Governor Thomas Dewey, Fish's congressional career ended when he won the Republican Party primary in his district but lost the general election in 1944.

He was named chair of the three-member committee which wrote the preamble to the Legion's constitution and selected two other Roosevelt supporters as its other members.

In November 1919, Fish attended the first American Legion convention in Minneapolis and backed the Roosevelt wing of the organization in its successful bid for control.

Fish instead appealed directly to voters on a fourteen-point platform which blended progressive domestic policy, opposition to internationalism, and reactionary nativism, including "restriction of immigration based on [a] character test" and "drastic action" against "Anarchists, I.W.Ws, Communists and ultra Socialists.

"[27] Despite opposition from the party establishment, Fish enjoyed high name recognition from his and his father's time in office and support from veterans, the state Federation of Labor, and Black voters.

He won the primary handily despite losing Dutchess County by 4,000 votes and very easily defeated Rosslyn Cox, the mayor of Middletown, in the general election.

After his victory, Fish received congratulations from Franklin Roosevelt, who had been defeated as the Democratic nominee for vice president in the concurrent 1920 election.

Fish also introduced House Resolution 67 of the 66th Congress, which provided for the internment of the remains of an unknown American soldier in a hallowed tomb to be constructed outside the Memorial Amphitheater in Arlington National Cemetery.

[29] Though Fish had earlier promised that the committee would request stronger immigration restrictions and deportation of communists,[30] his report stated, "the surest and most effective way of combatting communism in the United States is to give the fullest publicity to the fundamental principles and aims of the communist... as they are not likely to prove acceptable to any considerable number of American citizens, unless camouflaged by extraneous issues.

"[33] As the ranking Republican member on the United States House Committee on Foreign Affairs in the years leading up to World War II, Fish played the leading role in advocating against American involvement in the war and Roosevelt's policy of economic aid to the Allies, especially the Soviet Union, and what Fish viewed as antagonism of Germany and Japan.

[37] On August 14, 1939, Fish served as president of the United States delegation to the Interparliamentary Union Congress conference in Oslo, Norway, where he flew in the private plane of Joachim Ribbentrop.

[38] At the conference, Fish advocated better diplomatic relations with Nazi Germany and hoped to solve the "Danzig question," where he believed that German claims were "just.

In 1940, British agents established and operated the Nonpartisan Committee to Defeat Hamilton Fish to "put the fear of God into every isolationist senator and congressman.

"[43] In 1941, a judiciary panel investigating the activities of Nazi agents in the US, sent officers to the Washington headquarters of an anti-British organization, the Islands for War Debts Committee, to seize eight bags of franked congressional mail containing speeches by isolationist members of Congress.

[44] A grand jury was convened and summoned Hill to explain why he had been so solicitous about the Islands for War Debts Committee's mail and his close association with George Sylvester Viereck, a Nazi propaganda agent.

He published FDR: The Other Side of the Coin in 1976, arguing that Roosevelt had deliberately provoked the Japanese and had advance knowledge of the attack.

"[50] Less than two weeks before the 1942 election, Drew Pearson's nationally syndicated column Washington Merry-Go-Round described in detail how in 1939, Fish had received over $3,100 in cash from a source with German ties.

[53] As Time magazine reported, "In New York, to the nation's delight, down went rabid anti-Roosevelt isolationist Hamilton Fish, after 24 years in Congress.

"[55][56] Fish also stated, "I particularly wanted to be elected to serve as chairman of the Rules Committee to stop the march toward communism and totalitarianism in America.