After the October Revolution Semyonov stirred up an anti-Soviet rebellion, but was defeated and fled to the northeastern Chinese city of Harbin.
In August 1918 he managed to consolidate his positions in the Transbaikal region with the help of the Czechoslovak Legions and imposed his ruthless regime.
In his rule over this region, he has been described as a "plain bandit [who] drew his income from holding up trains and forcing payments, no matter what the nature of the load nor for whose benefit it was being shipped."
As a part Buryat Mongol, Ataman Semyonov declared a "Great Mongol State" in 1918 and had designs to unify the Oirat Mongol lands, portions of Xinjiang, Transbaikal, Inner Mongolia, Outer Mongolia, Tannu Uriankhai, Kobdo, Hulunbei'er and Tibet into one Mongolian state.
The White Siberian Provisional Government appointed Semyonov commander of a detached unit with headquarters in Chita.
Initially Adm. Aleksandr Kolchak refused to recognize Semyonov's authority, but he had no choice and had to accept Semyonov as de facto leader and confirm him as Commander-in-Chief of the Chita military district.
In early 1919 Semyonov declared himself  Ataman  of the Transbaikal Cossack Host with support from the Imperial Japanese Army, elements of which had been deployed to Siberia.
The region under his control, also called Eastern Okraina, extended from Verkhne-Udinsk near Lake Baikal to the Shilka River and the town of Stretensk, to Manzhouli, where the Chinese Eastern Railway met the Chita Railway, and northeast some distance along the Amur Railway.

Semyonov handed out copies of the Protocols of the Elders of Zion to the Japanese troops he became associated with.

After the fall of Kolchak's Siberian government, the admiral transferred power to Semyonov in the Far East.
However, Semyonov was unable to keep his forces in Siberia under control: they stole, burned, murdered and raped, and developed a reputation for being little better than thugs.
In July 1920 the Japanese Expeditionary Corps started a limited withdrawal in accordance with the Gongota Agreement signed with the Far Eastern Republic, undermining support for Semyonov.
Transbaikal partisans, internationalists and the 5th Soviet Army under Genrich Eiche launched an operation to retake Chita.
In October 1920 units of the Red Army and guerrillas forced Semyonov's army out of the Baikal region.
After having retreated to Primorye, Semyonov tried to continue fighting the Soviets, but was finally forced to abandon all Russian territory by September 1921.
After the October Revolution Semyonov stirred up an anti-Soviet rebellion, but was defeated and fled to the northeastern Chinese city of Harbin. In August 1918 he managed to consolidate his positions in the Transbaikal region with the help of the Czechoslovak Legions and imposed his ruthless regime. In his rule over this region, he has been described as a "plain bandit [who] drew his income from holding up trains and forcing payments, no matter what the nature of the load nor for whose benefit it was being shipped." As a part Buryat Mongol, Ataman Semyonov declared a "Great Mongol State" in 1918 and had designs to unify the Oirat Mongol lands, portions of Xinjiang, Transbaikal, Inner Mongolia, Outer Mongolia, Tannu Uriankhai, Kobdo, Hulunbei'er and Tibet into one Mongolian state. The White Siberian Provisional Government appointed Semyonov commander of a detached unit with headquarters in Chita. Initially Adm. Aleksandr Kolchak refused to recognize Semyonov's authority, but he had no choice and had to accept Semyonov as de facto leader and confirm him as Commander-in-Chief of the Chita military district. In early 1919 Semyonov declared himself  Ataman  of the Transbaikal Cossack Host with support from the Imperial Japanese Army, elements of which had been deployed to Siberia. The region under his control, also called Eastern Okraina, extended from Verkhne-Udinsk near Lake Baikal to the Shilka River and the town of Stretensk, to Manzhouli, where the Chinese Eastern Railway met the Chita Railway, and northeast some distance along the Amur Railway.
Semyonov handed out copies of the Protocols of the Elders of Zion to the Japanese troops he became associated with. 
After the fall of Kolchak's Siberian government, the admiral transferred power to Semyonov in the Far East. However, Semyonov was unable to keep his forces in Siberia under control: they stole, burned, murdered and raped, and developed a reputation for being little better than thugs. In July 1920 the Japanese Expeditionary Corps started a limited withdrawal in accordance with the Gongota Agreement signed with the Far Eastern Republic, undermining support for Semyonov. Transbaikal partisans, internationalists and the 5th Soviet Army under Genrich Eiche launched an operation to retake Chita. In October 1920 units of the Red Army and guerrillas forced Semyonov's army out of the Baikal region. After having retreated to Primorye, Semyonov tried to continue fighting the Soviets, but was finally forced to abandon all Russian territory by September 1921.
Grigory Mikhaylovich Semyonov