Hansard has reference to "Foreign Enlistment Bill" discussions from 1819 to 2006. Stephen presents late 19th century establishment views. Lorimer publishes the law of 1870 as it was originally enacted.
Numerous former members of the UK armed forces had fought in the South American wars of independence against Spain. In those conflicts, trained officers of what was then regarded as the strongest military in the world successfully organised insurgents against Spain and caused major headaches for the Spanish expeditionary forces. Ultimately, Spain lost most of her territorial possessions in the Western Hemisphere and Anglo-Spanish relations were left strained for several decades. A law was passed in 1819 to prohibit British subjects from participating in foreign wars, but during the American Civil War it was found to be ineffective.
On 19 July 1870 Napoleon III of France declared war on the Kingdom of Prussia. The outbreak of hostilities put the UK in a delicate diplomatic position. Although relations between the UK and France had steadily improved since the end of the Napoleonic Wars, the UK's relations with Prussia were also close at the time due to the then-recent marriage of Queen Victoria's eldest daughter to Prussian Crown Prince Frederick.
Following the French declaration of war public opinion in the UK was on the side of Prussia and her German allies, and in other European states (especially Italy) men attempted to volunteer in considerable numbers to fight on the Prussian side. In addition, contrary to the eventual result of the conflict, many neutral military observers thought the French Empire to be stronger than Prussia. Given such considerations, the British government was keen on maintaining neutrality in the conflict at all costs and wary of any actions by its subjects that might have antagonised Napoleon III.
The Act made it a crime for any subject of the United Kingdom of Great Britain and Ireland to enlist themselves in the military of any foreign power at war with any state with which the UK was at peace.