Andres or Andreu Nin was a leader of the Spainish workers movement who was among the founders of the Commuist Party of Spain. He consequently worked for the Communist International and Red International of Labour Unions (RILU or Profintern) in Russia and was on the staff of Leon Trotsky for a period. While in Russia he was won to the Left Opposition to Stalins ascending faction within the Russian Communist Party.
When he returned to Spain nin was instrumental in forming the Left Communists of Spain (ICE) the Bolshvik Leninist group affiliated to the International Left Opposition (ILO). However the ICE was a small group and largely isolated. Nin had a number of disagreements with Trotsky in this period specfically when Trotsky advised the ICE leader that entry into the Spainish Socialist Youth would augment the forces at their disposal. Nin disagreed and sought to forge a united party with the Workers and Peasants Bloc (BOC), a group coming out of the Communist movement but seen as being on its right wing. Eventually Nin broke with Trotsky and the ILO on this question and the fusion wn=ent ahead.
Nin (along with Joaquin Maurin) formed the Workers' Party of Marxist Unification (POUM) in 1935 as a Communist alternative to the Third International loyalist Communist Party of Spain.
After the Spanish Popular Front won the 1936 election Nin became councillor of justice. He supported the government but was far more radical than most of its members.
Nin later joined the devolved government in Catalonia, headed by Luis Companys but owing to Soviet demands was removed from office along with other POUM appointees.
Nin was unpopular with the Soviet government of Josef Stalin owing to his support of Leon Trotsky (indeed Nin had been Trotsky's secretary at one stage).
In June 1937 Nin and most of the leadership of POUM were arrested and sent to a Soviet camp near Madrid. Andrés Nin was executed on June 20, 1937.