It joined the Warsaw Pact on May 14, 1955 and activities in the alliance include the Warsaw Pact Invasion of Czechoslovakia. As early as May 1968, the BNA's Main Political Directorate launched a propaganda campaign to explain to the army the events in Czechoslovakia. Immediate military training began in early July, and by the end of the same month it was being carried out in Bulgaria under the control of Soviet officers. Two designated units (the 12th Motorized Rifle Regiment and 22nd Artillery Regiment led by Colonels Alexander Genchev and Colonel Ivan Chavdarov respectively) were then transferred to Soviet territory, where they passed under Soviet command. Military and ideological preparations ended on August 20, when the Council of Ministers decided that the country should take part in the intervention in Czechoslovakia.