Like many Cheka employees at the time, Blumkin was, politically, a Left Socialist-Revolutionary rather than a Bolshevik. Since this party was opposed to the Treaty of Brest-Litovsk, Blumkin was ordered by its executive committee to assassinate Wilhelm von Mirbach, the German ambassador to Russia; they hoped by this action to incite a war with Germany. This event was timed to occur at the opening of the Fifth All-Russian Congress of Soviets at the Bolshoi Theater in Moscow. On the afternoon of 6 July 1918, Blumkin – along with an aide, Nikolai Andreyev – went to the residence of the German Ambassador on Denezhny Lane. Blumkin gained entrance to the embassy by presenting forged documents. When Mirbach entered the drawing room, Blumkin pulled a gun from his case and shot the ambassador at point blank range, killing him. At the same time, the Left SRs launched a coup attempt in Moscow, which was quickly quelled. The members of the Left SR party at the Bolshoi Theater were arrested and the party was forcibly suppressed. Blumkin, however, escaped and went into hiding.