Sōhyō was much the largest of Japan's labour union federations throughout the period of its existence (1950-89). At the height of its influence it accounted for some 4,800,000 unionists in their constituent industrial unions (tansan). These included Kokurō (the national railwaymen's union), Nikkyōso (the JAPAN TEACHERS' UNION), Zendentsū (the electricians' union), Zentei (the communications workers' union), Shitetsu sōren (General Federation of Private Railway Workers' Unions of Japan), Jichirō (All-Japan Prefectural and Municipal Workers' Union) and Tekkō Rōren (Japan Federation of Steel Workers' Unions).
On its foundation in July 1950 it was regarded as a bulwark against far-left influences that had dominated, in particular, the CONGRESS OF INDUSTRIAL UNIONS (Sanbetsu kaigi). There is evidence of Occupation involvement in the process leading to its birth. Different constituent unions, however, had contrasting political views, and, in the turbulent ideological climate after the outbreak of the Korean War and the approaching Japanese peace settlement, Sōhyō soon moved towards the left. This was already evident in its second Congress in March 1951, which backed positions on a peace settlement and possible rearmament close to those of the left-wing Suzuki faction of the JAPAN SOCIALIST PARTY (JSP). After the JSP split into Left Socialist and Right Socialist Parties in October 1951, Sōhyō became the principal organisational support for the Left Socialist Party, strongly opposing US pressure on Japan to become part of an anti-Communist alliance. In July 1953, the Sōhyō Secretary-General, Takano Minoru, took the federation even further to the left. Reflecting on the international situation following the death of Stalin, Takano supported the 'peace forces', including the USSR and China, but excluding the United States. Takano once famously remarked that, being responsible for Sōhyō's birth, 'the Occupation hatched a chicken that turned into a duck'.
In April 1954, a substantial group of unions that opposed the turn to the left orchestrated by Takano defected from Sōhyō, and formed the ALL-JAPAN TRADE UNION CONGRESS (Zenrō Kaigi) (later the JAPAN CONFEDERATION OF LABOUR, or Dōmei), which became the principal federation of private-sector unions. Takano's far-left position, however, was under attack from those close to the left-wing factions of the JSP, so that he was eased out of the leadership and replaced by that of Ota Kaoru (Chairman from 1958) and Iwai Akira (Secretary-General from 1955). Ota and Iwai were Marxist-influenced and confrontational towards the revisionist KISHI Government of the late 1950s (and later governments), but hostile to the JAPAN COMMUNIST PARTY.
Their best-known achievement was the establishment of the annual 'spring struggle' from 1955. (This was initiated by Ota as Chairman of the Synthetic Chemical Workers Union.) They recognised that the great weakness of the Japanese enterprise union system was lack of co-ordination between the wage claims of thousands of individual unions.