The United Kingdom historically was the only European state with a volunteer army, and remained so through the first half of World War I. As the war dragged on into deadlocked trench warfare, the number of volunteers dried up, which led the government to introduce conscription under the Military Service Act. The Act exempted the whole of Ireland from conscription, and although an amending Act in 1918 empowered the government to extend conscription to Ireland, the extension was never implemented. Conscientious objectors were formally recognised under the Military Service Act, with the possibilities of absolute exemption from military service, exemption conditional upon performing civilian "work of national importance", or non-combatant service within the Army; some of those who refused the latter alternatives, or engaged in anti-war protests, went to jail. Conscription was ended in 1920.