On September 14, 1917, the Russian Provisional Government issued a declaration making Russia a republic. The monarchy had ended in March, when Nicholas II abdicated and his brother Michael refused the throne, and for six months the government had deliberately left the question of the country's form open, on the understanding that only a Constituent Assembly could decide it.
The declaration was a response to the Kornilov affair. At the end of August, General Lavr Kornilov, whom Kerensky had appointed commander in chief partly as a bulwark against the left and partly against the right, moved troops toward Petrograd. The accounts of who did what differ and have never been settled, but the effect was clear: the government had to call on the soviets and the armed workers of the capital to defend itself, and it emerged from the episode with its authority reduced and the Bolsheviks, who had done much of the organizing, restored to a position they had lost in July.
Declaring a republic was the government's answer. It let Kerensky present himself as the man who had saved the revolution from a military coup while keeping a form of words that satisfied no one. The monarchy was gone on paper; the Constituent Assembly had still not met, and its elections were repeatedly postponed.
What the declaration could not address was the state the country was in. The war was going badly and the army was disintegrating, with desertions running at enormous numbers by the autumn. Peasants were seizing land. They did not wait for legislation. The railways were breaking down, food supplies to the cities were failing, and inflation had made money nearly worthless. The government's authority outside Petrograd and Moscow was largely nominal.
It lasted another seven weeks. The Bolsheviks took power on the night of November 7, 1917. The rising cost few lives. It was mostly a matter of taking the telephone exchange, the railway stations and the bridges. Kerensky escaped and spent the rest of his life, another 53 years, arguing about it.
The Constituent Assembly finally met in January 1918, was dissolved by the Bolsheviks after one day, and Russia became a Soviet republic instead. The declaration of September 14, 1917 is the closest the country has come to being a republic under that name until 1991.