At the start of the war, the Irish Republican Army (IRA) split into two opposing camps: a pro-treaty IRA and an anti-treaty IRA. The pro-treaty IRA disbanded and joined the new National Army. However, because the anti-treaty IRA lacked an effective command structure and because of the pro-treaty forces' defensive tactics throughout the war, Michael Collins and his pro-treaty forces were able to build up an army with many tens of thousands of World War I veterans from the 1922 disbanded Irish regiments of the British Army, capable of overwhelming the anti-treatyists. British supplies of artillery, aircraft, machine-guns and ammunition boosted pro-treaty forces, and the threat of a return of Crown forces to the Free State removed any doubts about the necessity of enforcing the treaty. Lack of public support for the anti-treaty forces (often called the ''Irregulars'') and the determination of the government to overcome the Irregulars contributed significantly to their defeat.