In January 1918, when the Bolsheviks were divided over whether to sign the
Treaty of Brest-Litovsk with Germany, Smirnov joined the
Left Communists, led by Bukharin, who advocated 'revolutionary war' with Germany. In February, he resigned from the Bolshevik government to campaign against the treaty, and for the remainder of his life he was in opposition. Part of his reasoning was that there was that to attempt to build socialism in pre-industrialised Russia alone, "a side turning off the main highway of European socialism" was "foredoomed to failure." During the
civil war, he was a leader of the Military Opposition, who opposed the presence of thousands of former officers of the Imperial Army in the newly created Red Army. At the
8th Party Congress of the Russian Communist Party, Smirnov appeared as a delegate from the
5th Army. On 20 March 1919, he gave a speech to the Congress on the use of former Tsarist officers (termed "Specialists" within the party) and of
political commissars in the
Red Army. Responding to accusations from
Grigory Sokolnikov that he opposed the use of officers, which by this point had become a key part of Bolshevik military strategy, he denied favouring the use of
partisan militias in the
Russian Civil War. He did, however, warn of the inadequate political mechanisms that the Soviet authority had at its disposal to control the officer-specialists. Arguing for the repeal of
Decree on Revolutionary Military Councils, he said to the Congress: ...The role of the political commissars is limited to the functions of supervision... Now that we have the political commissars with sufficient combat experience and able not to intervene when not needed, we must give them broader rights, a larger part in the direction of the armies. Smirnov regarded the commissars as an integral check on the potential disloyalty of the old-régime officers. This preference for so-called "politicisation" of the Red Army was shared by the
Left Socialist-Revolutionary Party in opposition, but largely rejected by
Leon Trotsky, the
People's Commissar of Army and Navy Affairs, who by 1919 exercised full control over the military. But in April 1919 the Central Committee of the
RKP appointed Smirnov as the first organiser for
ChON volunteers to support the Red Army in the civil-war effort. During 1920, Smirnov, Osinsky, and
Timofei Sapronov formed the
Democratic Centralists, or 'Decists', a left wing opposition group that opposed the managerial system in industry, and advocated more democracy within the communist party. Smirnov signed
The Declaration of 46 in 1923, and acted as one of the main speakers for the opposition at the party conference in January 1924. In 1926, he and Sapronov formed the "Group of 15", which joined the
United Opposition headed by
Trotsky,
Grigory Zinoviev and
Lev Kamenev. ==Exile, prison and death==