In the early 1930s, Leon Trotsky and his supporters believed that Stalin's influence over the Third International could still be fought from within and slowly rolled back.
[1] Trotsky claimed that the Third Period policies of the Comintern had contributed to the rise of Adolf Hitler in Germany, and that its turn to a popular front policy (aiming to unite all ostensibly anti-fascist forces) sowed illusions in reformism and pacifism and "clear[ed] the road for a fascist overturn".
Three of those parties joined the Left Opposition in signing a document written by Trotsky calling for a Fourth International, which became known as the "Declaration of Four".
Fenner Brockway, ILP leader, was chairman of the Bureau for most of this period, while in 1939, Julián Gorkin of the POUM became its secretary.
By this time, the Bureau had member parties in more than 20 countries, including the Netherlands, Austria, Czechoslovakia, the United States, and Palestine.