When the Executive Committee of the Council of People's Commissars he joined Lenin, Trotsky and Stalin along with fellow Left SR, Vladimir Karelin.

At party forums and in the press, he sharply opposed the imperialist war, coalitions with the bourgeoisie, and demanded an early convocation of the Russian Constituent Assembly.

He took an active part in the preparation of October Revolution, but considered it undesirable to overthrow the Provisional Government before convening the All-Russian Congress of Soviets.

Along with his work in the People's Commissariat, he participated in the development of a decree on the organization of the Red Army and the Treaty between Russia and the Finnish Socialist Workers' Republic.

At the 2nd Left SR Congress (April 17–25, 1918, Moscow), Proshian strove for a breakdown of the world, unleashing a guerrilla-insurgent struggle against German imperialism.

According to Maria Spiridonova, Proshian was "an internal builder, the main spring, the core of the Left Socialist Revolutionary party".

Shortly after the congress, he became ill with typhus and was placed in a hospital under a false name (to avoid arrest); but all attempts to save Proshyan's life were unsuccessful, he died on December 16, 1918, and was buried at the Vagankovo Cemetery.