The Left faction of the Socialist Revolutionaries (the Left SRs) used to be allies of the Bolsheviks but now they are enemies. After murdering Count Mirbach, the German ambassador, the Left SRs have taken up arms and are calling on the masses to overthrow the Bolsheviks as betrayers of the revolution. Yesterday the situation hung in the balance, with Bolshevik forces in Moscow heavily outnumbered by the Left SRs, who controlled most of the armed militia of the Cheka, the political police.
But now the Left SR rising is over. The masses have failed to rally to the Left SRs while the rebels failed to capitalise on their temporary advantage by storming the Kremlin and arresting the Bolshevik leaders. Instead the uprising runs out of steam, with the Bolsheviks managing to round up Maria Spiridonova and other Left SR leaders (who have conveniently all gone to attend a meeting of the Soviet Congress without any armed guards) and muster enough forces of their own to oblige the surrender of the rebellious Cheka units.
With the Left SRs defeated, the Bolsheviks are once more secure in Moscow. Left SR supporters can now be purged from the Cheka and other Soviet bodies. The reformed Cheka will then be free to ensure there is no fresh challenge from the Left SRs or other disgruntled parties on the left.
Anarchist allies of the Left SRs (Alpha History: the Left SRs)
Pro-Bolshevik troops guarding the Bolshoi Theatre (Wikipedia: Aufstand der Linken Sozialrevolutionäre)