He was a key advisor to the 1934 Nye Committee, which investigated the role of the so-called "merchants of death" (munitions manufacturers and bankers) in leading to U.S. entry into World War I.

Both these passed in Congress, but America First gained enormous momentum in a nation riven by the entirely contradictory wishes to both support the British yet stay out of the war, with 850,000 members within a year of its founding.

In 1944 he wrote As We Go Marching, a sharp critique of the American drift toward statism of the fascist variety: But when fascism comes it will not be in the form of an anti-American movement or pro-Hitler bund, practicing disloyalty.

We are always moving forward with high mission, a destiny imposed by the Deity to regenerate our victims, while incidentally capturing their markets; to civilise savage and senile and paranoid peoples, while blundering accidentally into their oil wells.

[10]In fall 1944, Flynn completed a 25-page document entitled The Truth about Pearl Harbor[11] which so impressed publisher Robert McCormick that he had it printed in its entirety, beginning on the front page of the October 22 Chicago Tribune, in hopes of influencing the upcoming election.

[citation needed] Flynn also alleged that Pearl Harbor's able Navy and Army commanders, Admiral Husband E. Kimmel and General Walter Short, were left without proper warning that conditions had deteriorated to this extent with Japan, so they would be caught off guard.

Besides lowering his opinion of the GOP as too weak to fight his nemesis, Roosevelt, Flynn further reasoned that meant FDR must have known in precise detail that the attack was coming and deliberately withheld this information from the now-disgraced commanders in order to create an outraged demand for war.

An early critic of American involvement in the affairs of Indochina, he maintained that sending US troops would "only be proving the case of the Communists against America that we are defending French imperialism."

This was ironic because Flynn "had long ridiculed the idea that communism was a threat to America",[17] dismissing American Communists as a tiny handful of fractious, isolated radicals who were too busy attacking each other to threaten democracy.

For many years Flynn made his home in Bayside, New York in a large compound overlooking Little Neck Bay, with a house and a separate building he used as a broadcasting studio.