On 24 January 1919, the Organizational Bureau (Orgbureau) of the TsK VKP(b) (Central Committee of the All-Russian Communist party (bolsheviks) issued the document that started the politics known as the Decossackization (quoted from enotes.com):
24 January 1919
The latest events on different fronts in the Cossack regions — our advance into the interior of the Cossack settlements and the disintegration among the Cossack hosts — compels us to give instructions to party workers about the character of their work during the reestablishment and strengthening of Soviet power in the said regions. It is necessary to recognize, based on the experience of the civil war with the Cossacks, that the most merciless struggle with all the upper layers of the Cossacks through their extermination to a man is the only correct policy. No compromises or halfheartedness whatsoever are acceptable.
Therefore it is necessary:
- To carry out mass terror against wealthy Cossacks, exterminating them to a man; to carry out merciless mass terror in relations to all Cossacks have taken part in any way directly or indirectly in the struggle with the Soviet power. Against the middle Cossacks it is necessary to take all those measures which give a guarantee against any attempt on their part [to join] a new attack on Soviet power.
- To confiscate grain and force [them] to gather all surpluses in designated points; this applies both to grain and all other agricultural products.
- To take all measures assisting the resettlement of newly arrived poor, organizing this settlement where possible.
- To equalize newly arrived Inogorodnie with the Cossacks in land and in all other relations.
- To carry out complete disarmament, shooting those who after the time of handing over are found to have arms.
- To give arms only to reliable elements from the Inogorodnie.
- Armed detachments are to be stationed in Cossack stanitsas henceforward until the establishment of complete order.
- To order all commissars appointed to this or that Cossack settlement to show maximum firmness and to carry out the present orders unswervingly.
TsK imposes the obligation on Narkomzem to work out quickly practical measures concerning the mass resettlement of poor on Cossack land to be carried out through the corresponding soviet institutions.
It is generally believed that the directive was written by Sverdlov and Tsyurupa. Some historians, though, question this idea, arguing that the protocols of the TsK VKP(b) do not correspond to this directive. So, G. Magner wrote in "The Decossackization in the system of mass repressions" (in Russian) that the author was none other than Stalin, who took this decision alone.
Cossacks are a social, not ethnical (probably, sub-ethnical), group or Russian population. They have a long and complicated history. Basically, they are the descendants of free people who left Russia for the new lands and were later recognized as a military organization and performed the functions of border guards for centuries. The Inogorodnie (newcomers or, literally, people from other towns) arrived to the lands settled by Cossacks later, mostly in late XIX century, when Russian government encouraged the people to move to these lands. They did not have to carry out the military duties and could concentrate on agriculture. Cossacks were proud of their outstanding position in Russia — free people and valued warriors. So, they discriminated the Inogorodnie who naturally responded with hatred. So, when the Civil war began, the Inogorodnie supported the bolsheviks. Contrary to the popular belief, the Cossacks did not unanymously support the Whites. During the first year, the Cossacks preferred to stay aside from the Civil war and refused to provide assistance to the Volunteer Army of general Alexeyev. However, they did not try too hard to drive the Volunteer Army from the Cossack lands, where it was formed. Later, the Cossacks split in three parties. Some of them supported the bolsheviks, others fought in the White Guard, while the remaining large group formed independent guerilla groups (in many occasions the bandits would be a better word), called the Greens. In January 1919 seven Cossack stanitsas (stanitsa is a small town or a large village) rebelled against the ataman (the Cossack leader) P. Krasnov. They sent messengers to the Red army and then joined the bolsheviks.
The Wikipedia article on decossackization quotes a number of books that call the decossackization genocide. While very close to the truth, this word does not describe the situation correctly. Modern historian G. Pomerants proposed the term "stratacide", the extermination of a social stratum. The bolsheviks did strive to eliminate the Cossack culture. They opposed, and opposed fiercely, their traditions, including the military uniform. The villages were renamed and even the word "Cossack" was banned. The Cossacks were persecuted with the extreme atrocity. There are many documents written by the bolshevik leaders, including Lenin, Trotsky and Stalin, with the direct orders to exterminate the richer Cossacks, the atamans (the leaders), teachers, clerics, etc. In December 1919 Dzerzhinsky wrote a letter to Lenin asking him what to do with the captive Cossacks, there were about one million of them. "Shoot them all," replied Lenin. The Inogorodnie who hated the Cossacks often used these orders as an occasion to exterminate the Cossacks indiscriminately. Even the Cossacks who collaborated with the bolsheviks could not be safe. And yet, thousands of the Red Cossacks who fought together with the bolsheviks played an important role in the Civil war.
As a response to these prosecutions, the Cossacks revolted in March 1919 and many of them joined the Volunteer Army. The bolsheviks backed down. They condemned the "excesses" of the decossackization and in 1920 ran the All-Russian Congress of the working Cossacks. By mid-20s the decossackization was basically over.
I have recently finished reading the book "Russia. My native land" by Gregory P. Tschebotarioff, professor of the Princeton University, former Cossack officer. I enjoyed this simple, plain and honest book. Tschebotarioff says that he wrote the book to disprove the false conceptions common in the West. He tries to explain to the Americans what the real Russia looked like before the revolution, who are the Cossacks and what happened in 1917. Tschebotarioff knew personally many leaders of the White movement and the Cossack leaders — general Denikin, ataman Krasnov, general Kornilov and others. Tschebotarioff rejects the idea that the Cossacks were separatists who wanted to secede from Russia. When in 1959 the US Congress adopted the US Public Law 86-90 (The Law on Enslaved Peoples, if I back-translate the name correctly), obliging the United States to assist the peoples subjugated by the bolsheviks. In January 1961, Tschebotarioff wrote an article protesting again the false ideas on which the law was based. So, he says that the law offers assistance to the mythical "countries" like Cossackia or Idel-Ural, while there have never been such countries. He insists that the Cossacks were never separatists and that the words of ataman Krasnov and other Cossack leaders were misinterpreted. Being a Cossack and knowing all these people personally, he is very convincing. I highly recommend this book to all students of Russian history. A thorough, deeply personal account, it lacks the analytical approach, but describes with extreme honesty the real events of those years.