In August 1914 the Second International grouping of socialist parties failed its most important test with catastrophic consequences.
Nearly all the leaders of European socialism collapsed into chauvinism, supporting their own nations' interests in an imperialist war which cost the lives of tens of millions of workers.
One of the few parties to remain against the war throughout was the Bolsheviks in Russia. The experience of war and disillusionment with their leaders led to the radicalisation of workers and soldiers.
After revolution in October 1917, the Bolsheviks led a government of soviets and pulled Russia out of the war. In November 1918 revolution erupted in Germany which finally ended the war.
Lenin saw that a new International was needed, grouping the best elements of the socialist parties in Europe. The Communist International, or Comintern, was launched in early March 1919. The greatest challenge was that, other than the Bolsheviks, the affiliated parties were small and not yet tempered in the flame of revolution.
Yet a revolutionary conflagration was sweeping Europe. Soviet republics were proclaimed in Budapest on 21 March 1919 and in Munich on 7 April and there were strike waves in Austria, Italy and Britain.
Lenin saw the opportunity to draw mass parties under the revolutionary socialist banner.
He was proved right. The Italian Socialist Party of 300,000 members joined, as did mass parties from Bulgaria, Yugoslavia, Norway and Sweden. The Czechoslovak party soon counted 400,000 members.
In Germany the Independent Social Democratic Party (USPD) had split from the SPD in 1917. Two years later it split again with 300,000 members joining the Communist Party (KPD). In France 140,000 members of the Socialist Party joined the newly formed Communist Party.
But these new parties were inexperienced in revolutionary politics. The KPD was immediately drawn into a disastrous, premature uprising in Berlin in January 1919 and the soviet regimes lasted only four months in Hungary and three weeks in Bavaria.
The tide was turning and lessons had to be drawn. In the run-up to the Second Congress of the Comintern in 1920, at a time when the Civil War had not been finally won against the counter-revolutionary "whites" in Russia, Lenin took time to make a survey of the European parties and their readiness for the next phase of the revolution.
The result was the pamphlet "Left Wing" Communism: an Infantile Disorder. It was to be one of his most important writings. It reads like a text book of strategy and tactics for a revolutionary party. Lenin drew on the rich experience of the Bolsheviks over more than two decades, through periods when the movement was advancing and retreating, to help guide the impatient revolutionaries in the new Communist parties.
The "infantile disorder" of these young parties took many forms. Lenin acknowledged the importance of principles and discipline but he also stressed the need for tactical flexibility and sometimes "compromises" in order to be in the best position to win the greatest number of workers to revolution.
"Politics is a science and an art that doesn't fall from the skies", he wrote. "If it wants to overcome the bourgeoisie, the proletariat must train its own class politicians."
At its founding conference the German KPD had voted against Rosa Luxemburg's advice and decided to boycott the parliamentary elections, counterposing it to building soviets.
Lenin argued that as long as workers have illusions in parliament the party is obliged to participate in elections in order to free the masses from those illusions. He made the point that even after the October Revolution the Bolsheviks took part in elections to the Constituent Assembly which yielded "exceedingly valuable (and to the proletariat, highly useful) political results."
Delegates to the first KPD congress also voted to leave the trade unions, with one leading member raising the ultra-left slogan "Out of the unions" and calling for revolutionary workers' unions. These positions separated off the KPD from the mass of workers who were in a process of radicalisation.
Lenin wrote, "To refuse to work in the reactionary trade unions means leaving the insufficiently developed or backward masses of workers under the influence of the reactionary leaders."
Throughout the pamphlet Lenin gave specific advice, and sometimes stinging criticism, to each of the new sections of the International. This is never through pettiness but because the stakes were so high.
Every worker's movement reflects the uneven consciousness of the class. There are militant minorities ready to break from the old reformist leaders and trade union officials but others, often the majority, still under their influence.
Left Wing Communism is a manual that seeks to find ways for the vanguard to avoid separating themselves off from the masses and increase their influence over a wider number until they have won over the majority. It remains a powerful weapon in our hands today.