On September 3rd, 1938, at a one-day Congress facing the terrible persecution under Stalin persecution, the Fourth International was founded. Thus ended a long battle against the bureaucratic degeneration that Trotsky began in 1923 and that Lenin himself had started a year earlier.
The confrontation of Stalinist terror and the defense of USSR
Since 1933, when the Third International supported the policy developed by the German Communist Party that led Hitler to triumph and to the worst defeat of the German proletariat, Trotsky comes to the conclusion that there is not any task more urgent than founding the Fourth International.
That was the only way of preserving the Leninist principles and of being ready for the next revolutionary rise up that, most likely, would come after the next world war.
It was not an easy task. On one hand, Nazism advanced furiously. On the other hand, Stalinism launched the policy of Popular Front to face fascism, it means, a front with the “democratic” bourgeoisie; before this popular front launching Stalin had denied the United Front of the working class to confront Hitler. Added to this, there was the counter-revolutionary attack against all that was left from the old Bolshevik leadership.
With the horrific Moscow trials, on false charges, was physically wiped out most of the Bolshevik leadership who participated in the seizure of power and who had faced Stalin terror being these attacks from the left and from right. Thus tumbled Zinoviev, Kamenev, Bukharin… At the same time Trotsky followers (among those, Trotsky’s own children) were dying in USSR’s concentration camps or under the action of mercenaries that chased them throughout Europe.
The Stalinist barbarism engendered a current within the movement pro-Fourth International, which began to propose there was no reason to defend the USSR, as no different from imperialism. Amid the Stalinist attack, Trotsky developed a relentless battle against this anti-defensist tendency proposing the unconditional defense of the USSR before any imperialist attack. At the same time, Trotsky said that the only way to defend October gains was carrying out a political revolution that took away the power of bureaucracy.
The battle against the sceptics
Although Trotsky had been criticized for ten years by those who opinioned that there was no need to struggle for a reform in the Soviet Communist Party and in the Third International; now (September 1938) the majority of his followers were not convinced that the Fourth International should be founded. These followers’ main justifications were very similar to those that nowadays many trends use to justify not building the International: “that the moment hadn’t come yet”, “that they would build something very fragile”, “that great class struggle facts that could justify the International building hadn’t happened yet”… Trotsky answered to those arguments through great amount of letters. In the Transitional Program, he does so once again.
“The skeptics ask: But has the moment for the creation of the Fourth International yet arrived? It is impossible, they say, to create an International “artificially”; it can arise only out of great events’, etc., etc. (…) The Fourth International has already arisen out of great events: the greatest defeats of the proletariat in history”. The cause for these defeats is to be found in the degeneration and perfidy of the old leadership. The class struggle does not tolerate an interruption. The Third International, following the Second, is dead for purposes of revolution. Long live the Fourth International!
But has the time yet arrived to proclaim its creation? … the skeptics are not quieted down. The Fourth International, we answer, has no need of being “proclaimed.” It exists and it fights. Is it weak? Yes, its ranks are not numerous because it is still young. They are as yet chiefly cadres. But these cadres are pledges for the future.
Outside these cadres there does not exist a single revolutionary current on this planet really meriting the name. If our international be still weak in numbers, it is strong in doctrine, program, tradition, in the incomparable tempering of its cadres.” 
The Foundation of the Fourth International
Finally, in September 3rd 1938, the Fourth International is founded in Paris. Due to security issues caused by Stalinist terrorism Trotsky does not participate in this congress. A couple of days before the foundation meeting, Rudolf Klement, Trotsky’s secretary had been kidnapped and murdered by GPU. Klement was responsible for organizing the foundational congress. For the same reason the congress would take only one day and would vote just a few documents: the Transitional Program, a sketch of the statute to be informed orally (the original version had vanished along with Klement), a manifesto against the war, a resolution on youth as well as a greeting letter to Trotsky, to the murdered comrades and to the combatants of the Spanish Civil War.
There were delegates from Soviet Union, Great Britain, France, Germany, Poland, Italy, Greece, Holland, Belgium and United Sates and one delegate representing Latin America (the Brazilian Mario Pedrosa).
All those countries organizations were small and Trotsky would say: “Our situation now is incomparably more difficult than that of any other organization in any other time (…) Nothing in the world is so convincing as success and nothing so repelling as defeat for the large masses. (…)
You have also the degeneration of the Third International on the one side and the terrible defeat of the Left Opposition with the extermination of the whole group (…) Also, the social composition of every revolutionary movement in the beginning is not of workers. (…) We are all very critical toward the social composition of our organization and we must change, but we must understand that this social composition did not fall from heaven, but was determined by the objective situation and by our historic mission in this period.” 
Nahuel Moreno  used to say that the Fourth International had born with a giant head in a dwarf’s body. Stalin was aware of the giant head’s power, who synthesized the experience of the three Russian revolutions. That is why he recklessly sought to cut down Fourth’s “head”. He eventually did it. On August 20th, 1940 one of his agents, Ramón Mercader, assassinated Trotsky. Trotsky’s death was a severe blow to the newly founded Fourth International.
The dispersion of the Fourth and the need to rebuild it
The loss of the giant’s head left the Fourth in appalling conditions to face the Second World War, the combined attack of Nazism and Stalinism and the great changes of the postwar period. The weakness and inexperience of Fourth’s leaders led them to fall into sectarian deviations at first, then surrender to counterrevolutionary apparatuses that were strengthened with the outcome of the war, causing a dispersion process that continues today.
This leads to the contradiction that today, when the masses of Eastern Europe have given the decisive stroke to the Stalinist apparatus, the Fourth as an organization does not exist, even though its program has been confirmed by reality.
There is no task more important and urgent than to rebuild the Fourth; as Trotsky said, “without leadership, without an international leadership, the proletariat can not liberate himself from the current oppression”. 
We must face the reconstruction not using the methodology of self-proclamation or through the holding of speeches and open conferences, as do many sects that claim to be Trotskyites. We must accomplish this task with the same method applied by Trotsky in its construction: no self-proclamations and calling the revolutionaries to take together the revolutionary struggle and the programmatic discussion.
We must face patiently these programmatic discussions, without ultimatums, but no diplomacy, in front of the masses and not forgetting other principles of revolutionary politics: “Do not be scared unnecessarily, and do not scare the others without a reason. Do not make false accusations, do not label as “capitulation” where there is no capitulation, and do not replace the Marxist discussion by the disputes without principles”. 
 Trotsky, Transitional program, www.marxists.org
 Trotsky, Fighting against the stream, April 1939, www.marxists.org
 Nahuel Moreno: Historical Argentinean Trotskyite, he was founder of the Lit-CI and its main leader. He died in 1987.
 Bolshevik-Leninists statement to the Communist and Left-socialist Organizations Conference, 8-17-1933, http://marxists.org/espanol/trotsky/ceip/escritos/libro3/T05V109.htm#_ftn1
 León Trotsky, Algo más sobre la Conferencia de París, 10-12-1933, http://www2.cddc.vt.edu/marxists/espanol/trotsky/ceip/escritos/libro3/T05V121.htm