Despite its name the Siberian Host was based in quite a small area along the Trans-Siberian Railroad from Omsk to Semipalatinsk, with isolated lands scattered around, including Akmolinsk. It was never numerous.
Due to its location in a crucial theatre, with Omsk being the capital of Siberia, and on the railroad, Siberian Cossacks became fully involved in the Civil War from the very start. However the Siberian Host as a whole did not mobilise properly, and their efforts pale beside those of the Don, Astrakhan, Urals or Orenburg.
The Siberian Host soldiers wore standard Cossack dress, with their distinguishing colour being red (often given as "scarlet").
Few Siberian Cossacks served as other than cavalry, as the Independent Siberian Army and then Kolchak's Siberian Army were desperately short of mounted troops.
From the start the Omsk region was never particularly supportive of Lenin's takeover. A large anti-Bolshevik underground formed almost immediately, with Siberian Cossacks being a main element. Some famous names in the White cause – Colonels Grishin-Almazov, Ivanov-Rinov, V. Volkov and "Ataman" B. Annenkov – were Siberian Cossacks active in early 1918. They linked up with officers and students, but were reluctant to work alongside a parallel Socialist-Revolutionary underground, as their politics were reactionary. Attempted risings in the main Siberian towns failed through disorganisation and lack of unity, despite the weakness of the Reds.
When the Czechs revolted these Whites quickly rallied to the cause. After some muddled initial attempts at government, by July the Siberian Provincial Government was organised in Omsk. This was a right wing organisation from the start, and became more so over time. It assembled an army, but for short-sighted political reasons would not assist the KOMUCH government actively fighting the Red Army with the Czechs to their immediate west. Instead they got into an active squabble about who was to control the central Ural Mountains, diverting precious effort from the main fighting.
The new Siberian Army was largely led by Siberian Cossacks in the early stages, as they were the only trained men available. Grishin-Almazov was initially in command, but in August was replaced by the new Ataman of the Siberian Host, General Ivanov-Rinov. The new commander immediately brought back traditional military insignia and a very reactionary attitude in general.
The collapse of the KOMUCH People's Army at the end of 1918 brought about the short-lived Ufa Directory, but still the Siberian units did not move east. Nor did the Siberian Host mobilise fully. Luckily for them the People's Army commander, General Voitsekhovskii, persuaded the Czechs to put in one last effort. In November 1918 they opened a gap by attacking north of Belebei, which allowed the People's Army's best formations (Kappel' Combat Team, Molchanov's Brigade, Izhevsk and Votinsk Workers Regiments) to reach safety in Ufa.
By this stage Kolchak had taken power in Omsk, backed by many Siberian Cossacks. In Spring 1919 he began his drive westwards, initially with great success. The handful of Siberian Cossack regiments, two divisions' worth, were brigaded with the White units to provide cavalry support. It did not last long, and by mid-1919 the Red Army was counter-attacking with great vigour, and the White Army was pulling back behind the Ural Mountains.
Ataman Ivanov-Rinov had been trying to organise a Cossack Corps as reinforcements, as the Host finally realised that they would need to pull their weight if the White cause was to have any chance. Even then they did not call up particularly many age groups, and only had some 7,500 sabres in August 1919. Three new Siberian Cossack Divisions brought the total up to five, each meant to be of three cavalry regiments, an artillery divizion (3 batteries), an engineer divizion and a plastoon battalion (4 sotnias). In addition there was a Separate Cossack Brigade of three divizions (each 3 sotnias).
Not all these forces could be sent to the main front, as partisan groups increasingly filled the rear. In the end the Siberian Corps moved some 4,500 sabres, 50 MGs and half-dozen guns for its one and only campaign. It was tasked in the autumn offensive with outflanking Tukhachevskii's 5th Army from the south, taking Kurgan and preventing a Soviet retreat. Casualties were heavy on both sides and Ivanov lost his nerve, halting on the east bank of the Tobol River at the end of September. Although they had inflicted a serious defeat on the face of it, capturing much equipment and driving the enemy back hundreds of kilometres, their halt allowed the remaining Reds to retreat more or less unimpeded. The Soviets soon recovered and attacked again.
From this time onwards the Siberian Host lands were fully conquered, and the men either surrendered or followed Kolchak's remaining armies in the "Ice March". Quite a few made it to TransBaikal, and they were merged into a brigade and a couple of regiments. However their numbers fell quite quickly, and it seems likely that many returned home during 1920 and 1921.