Conscientious objector
Conscientious objectors formed only a tiny proportion of Military Service Tribunals' cases over the conscription period, estimated at 2 per cent. Tribunals were notoriously harsh towards conscientious objectors, reflecting widespread public opinion that they were lazy, degenerate, ungrateful 'shirkers' seeking to benefit from the sacrifices of others.
In an attempt to press the issue, in May 1916 a group of thirty-five objectors, including the Richmond Sixteen, were taken to France as conscripts and given military orders, the disobedience of which would warrant a death sentence.  These men, known as "The Frenchmen", refused; the four ringleaders were formally sentenced to death by court-martial but immediately reprieved, with commutation to ten-years' penal servitude.
Although a few objectors were accepted for non-combatant service in the Royal Army Medical Corps, acting as nursing/paramedic assistants, the majority of non-combatants served in the Non-Combatant Corps on non-lethal stores, road and railway building and general labouring in the UK and France. Conscientious objectors who were deemed not to have made any useful contribution to the state were formally disenfranchised (through a clause inserted in the Representation of the People Act 1918 at the insistence of back-bench MPs) for the five years 1 September 1921 – 31 August 1926, but as it was a last-minute amendment there was no administrative machinery to enforce it, which was admitted to be a "dead letter".
Britain's conscription legislation of 1916 did not apply to Ireland, despite it then being part of the United Kingdom. In 1918 the Army's manpower shortage led to passing a further act enabling conscription in Ireland if and when the government saw fit. In the event, the government never saw fit, although the legislation led to the Conscription Crisis of 1918. British conscription in the Second World War did not apply to Northern Ireland. Many Irishmen volunteered to fight in both world wars. The various parts of the British Empire and Commonwealth had their own laws: in general, all the larger countries of the Empire participated, and some were, in proportion to their population, major participants in the First World War.
In the Second World War, following the National Service (Armed Forces) Act 1939, there were nearly 60,000 registered Conscientious Objectors. Testing by tribunals resumed, this time by special Conscientious Objection Tribunals chaired by a judge, and the effects were much less harsh. If objectors were not a member of the Quakers or some similar pacifist organisation, it was generally enough to say that they objected to "warfare as a means of settling international disputes", a phrase from the Kellogg–Briand Pact of 1928. The tribunals could grant full exemption, exemption conditional on alternative service, exemption only from combatant duties, or dismiss the application. Of the 61,000 who were registered, 3,000 were given complete exemption; 18,000 applications were initially dismissed, but a number of such applicants succeeded at the Appellate Tribunal, sometimes after a "qualifying" sentence of three-months' imprisonment for an offence deemed to have been committed on grounds of conscience. Of those directed to non-combatant military service almost 7,000 were allocated to the Non-Combatant Corps, re-activated in mid-1940; its companies worked in clothing and food stores, in transport, or any military project not requiring the handling of "material of an aggressive nature". In November 1940 it was decided to allow troops in the NCC to volunteer for work in bomb disposal and over 350 men volunteered. Other non-combatants worked in the Royal Army Medical Corps. For conscientious objectors exempted conditional upon performing civil work, acceptable occupations were farm work, mining, firefighting and the ambulance service. About 5,500 objectors were imprisoned, most charged with refusal to attend a medical examination as a necessary preliminary to call-up after being refused exemption, and some charged with non-compliance with the terms of conditional exemption. A further 1,000 were court-martialled by the armed forces and sent to military detention barracks or civil prisons. Unlike the First World War, most sentences were relatively short, and there was no pattern of continually repeated sentences. The social stigma attached to 'conchies' (as they were called) was considerable; regardless of the genuineness of their motives, cowardice was often imputed.
Conscription in the United Kingdom was retained, with rights of conscientious objection, as National Service until the last call-up in 1960 and the last discharge in 1963. The use of all volunteer soldiers was hoped to remove the need to consider conscientious objectors. Ever since the First World War, there have been volunteer members of the armed forces who have developed a conscientious objection to service; a procedure was devised for them in the Second World War and with adaptations, it continues.

United States: There are currently legal provisions in the United States for recognizing conscientious objection, both through the Selective Service System and through the Department of Defense. The United States recognizes religious and moral objections, but not selective objections. Conscientious objectors in the United States may perform either civilian work or noncombatant service in lieu of combatant military service.
Historically, conscientious objectors have been persecuted in the United States. After the Selective Service System was founded during World War I, such persecutions decreased in frequency, and recognition for conscientious objectors grew.

Other countries: As of 2005, conscientious objectors in several countries may serve as field paramedics in the army (although some do not consider this a genuine alternative, as they feel it merely helps to make war more humane instead of preventing it). Alternatively, they may serve without arms, although this, too, has its problems. In certain European countries such as Austria, Greece and Switzerland, there is the option of performing an alternative civilian service, subject to the review of a written application or after a hearing about the state of conscience. In Greece, the alternative civilian service is twice as long as the corresponding military service; in Austria Zivildienst is one-third times longer, the Swiss Zivildienst is one and one-half times longer than military service. In 2005, the Swiss parliament considered whether willingness to serve one and a half times longer than an army recruit was sufficient proof of sincerity, citing that the cost of judging the state of conscience of a few thousand men per year was too great.

Conscientious objection in professional forces: Only two European Union countries – Germany and the Netherlands – recognize the right to conscientious objection for contract and professional military personnel.
In the United States, military personnel who come to a conviction of conscientious objection during their tour of duty must appear in front of a panel of experts, which consists of psychiatrists, military chaplains and officers.
In Switzerland, the panel consists entirely of civilians, and military personnel have no authority whatsoever. In Germany, the draft has been suspended since 2011.

See also: 

References: 

Further reading: Alexander, Paul, (2008),Peace to War: Shifting Allegiances in the Assemblies of God. Telford, PA: Cascadia Publishing/Herald Press. A history and analysis of conscientious objection in the Assemblies of God, the largest Pentecostal denomination.
Barker, Rachel. Conscience, government and war: Conscientious objection in Great Britain 1939–45 (Routledge, 2021).
Bennett, Scott H. (2003). Radical Pacifism: The War Resisters League and Gandhian Nonviolence in America, 1915–1963. (Syracuse Univ. Press).
Brennan, Jason. "Conscientious tax objection: why the liberal state must accommodate tax resisters." Public Affairs Quarterly 26.2 (2012): 141–159.
Keim, Albert N. (1990). The CPS Story: An Illustrated History of Civilian Public Service, pp. 75–79. Good Books. ISBN 1-56148-002-9 online
Gingerich, Melvin (1949), Service for Peace, A History of Mennonite Civilian Public Service, Mennonite Central Committee.
Krahn, Cornelius, Gingerich, Melvin & Harms, Orlando (Eds.) (1955). The Mennonite Encyclopedia, Volume I, pp. 76–78. Mennoniite Publishing House.
Magelssen, Morten. "When should conscientious objection be accepted?"  Journal of Medical Ethics 38.1 (2012): 18–21. online
Matthews, Mark (2006). Smoke Jumping on the Western Fire Line: Conscientious Objectors during World War II, University of Oklahoma Press. ISBN 978-0806137667
Moorehead, Caroline (1987). Troublesome People: Enemies of War, 1916–86, Hamish Hamilton Ltd, ISBN 0-241-12105-1
Quakers in Britain — Conscientious Objectors.
Smith, C. Henry (1981). Smith's Story of the Mennonites. Revised and expanded by Cornelius Krahn. Newton, Kansas: Faith and Life Press. pp. 299–300, 311.