The Don Republic (Донская республика) was a short lived state, that lasted for about a month in 1917 and 1918.
After the failed Kerensky-Krasnov uprising in late 1917 against the Bolsheviks, Cossack General Pyotr Krasnov fled south. While Alexander Kerensky went to the city of Tsaritsyn to begin rallying support for his newly created White movement, Krasnov went to the Don historical host, where he proclaimed the creation of a Don Republic, on the 2nd of December, 1917. As the Ataman of the Republic, he amassed a militia force to prepare to aid the white forces of General Anton Denikin, as the Russian Civil War was beginning. However, in January 1918, he agreed to dissolve the Republic and make it part of the newly formed Provisional All-Russian Government, which was formed at the Conference of Tsaritsyn.
Krasnov managed to put together a militia force of some 11,000 Don Cossacks to be the "Don Army", which was made part of the Russian National Army (the unified White forces) after the Republic itself was made part of the provisional government.