Nor was any action taken against the Irish and Quebec Catholics (British subjects) who fought on behalf of the Papal States (as Zouaves) in the late 1860s, during the Italian Risorgimento.

[1] In the small number of prosecutions, the Canadian courts rejected all jurisdiction challenges raised by the accused, concluding that the imperial act applied in British North America.

[1] The highest-profile case was that of Colonel Arthur Rankin, a militia officer of the Legislative Assembly of the Province of Canada who in the summer of 1861 personally offered his services to Abraham Lincoln, accepted a Union commission and raised the First Michigan Lancers.

[1] Parliament formally suspended the law on one occasion – in 1835, during the First Carlist War, for two years, permitting British subjects to join the army of Queen Isabella II of Spain.

[1][6] Problems with evidence prevented the British government from convicting enlistees to the French Foreign Legion or those thousands who joined the fight against Francisco Franco in the Spanish Civil War.

[9] In 2017, a British man who had joined the Russian separatist forces in Ukraine was convicted under terrorism laws, receiving a jail sentence of five years and four months.

Former Attorney General for England and Wales Dominic Grieve, a former member of Truss's Conservative Party, said that Britons who fought in Ukraine would be violating the 1870 Act.

By contrast, Sir Bob Neill, the chair of the House of Commons' Justice Select Committee, called the Foreign Enlistment Act an "antiquated piece of legislation" that should not be enforced.

He also noted that section 4 of the act does not extend to "enlistment in a foreign government's forces which are engaged in a civil war or combating terrorism or internal uprisings.

[1] The bill went through a first and second reading but was dropped by Prime Minister Alexander Mackenzie, who cited a lack of necessity for the law and expressed concerns that it might conflict with the imperial act.