Who were they?
The Mensheviks were a faction of the Russian revolutionary movement that emerged in 1903 after a dispute between Vladimir Lenin and Julius Martov, both members of the Russian Social Democratic Labour Party. At the 2nd Congress of the RSDLP, Lenin argued for a small party of professional revolutionaries with a large fringe of non-party sympathizers and supporters. Martov disagreed, believing it was better to have a large party of activists. A majority of party members agreed with Martov and formed the Mensheviks, while Lenin's faction became known as the Bolsheviks. Although a majority of rank and file party members agreed with Martov, they formed a minority among the party leadership, and hence Menshevik is a Russian word meaning "minority" while Bolshevik means majority. They were often also called "The Whites", as opposed to "The Reds".
The Mensheviks swung to the left in the 1905 Revolution and were particularly active in the Soviets and the emerging trade union movement. This had the result that they reunited with the Bolshevik faction and the RSDLP was able to function as a single grouping for some years.
The split between the two factions was long standing, and had to do both with pragmatic issues based in history such as the failed revolution of 1905, and theoretical issues of class leadership, class alliances, and bourgeois democracy. Both factions believed that Russia was not developed to a point at which socialism was possible and believed that the revolution for which they fought to overthrow the Tsarist regime would be a bourgeois democratic revolution. The Bolsheviks felt that the working class should lead the revolution in an alliance with the peasantry with the aim of establishing the democratic dictatorship of the proletaria and the peasantry, where the Party acts as extreme revolutionary opposition. On the other hand, the Menshevik vision was one of a bourgeois democratic revolution in which they could take part in government.
After the Split
Many Mensheviks left the party after the defeat of 1905 and joined more legal opposition organisations. After a while, Lenin's patience wore out with their compromising and in 1908 called the Mensheviks "liquidationists" and this eventually led to the Bolsheviks declaring their faction to be the party in 1912 with the aid of a handful of Mensheviks. The Menshevik faction proper further split in 1914 with the advent of the war. Most Mensheviks opposed the war, but a vocal right-wing minority supported it in terms of "national defence". After the revolution of 1917, most Mensheviks supported the war effort under the slogan "defence of the revolution". The growing left wing of the party, led by Martov, was strongly critical of this position, and was completely aghast at the party's decision to join a bourgeois socialist coalition government.
Lenin's more radical positions grew in popularity during the First World War as anger mounted against the czarist regime, and a number of leading Mensheviks such as Alexandra Kollontai joined the Bolsheviks as did Leon Trotsky and the grouping which supported him called the Mezhrayonka.
This split in the party crippled their popularity, and they received less than 3% of the vote compared to the Bolsheviks' 20%. The right wing of the Menshevik party supported right-wing actions against the Bolsheviks, while the left wing, the majority of the Mensheviks at that point, supported the Left in the ensuing Russian Civil War. However Martov's leftist Menshevik faction refused to break with the right wing of the party with the result that their press was sometimes banned and only intermittently available.
Menshevism was finally made illegal after the Kronstadt Uprising of 1921. A number of prominent Mensheviks emigrated thereafter. Martov who was suffering from ill health at this time went to Germany, where he died in 1923. However before his death he established the paper Socialist Messenger reportedly with funds supplied by his old comrade Lenin. The Socialist Messenger would move along with the Menshevik centre from Berlin to Paris in 1933 and then in 1939 to New York where it was to be published up until the early 1970s.