17th and 18th centuries
During the 17th and 18th century extensive use was made of foreign recruits in the now regimented and highly drilled armies of Europe, beginning in a systematized way with the Thirty Years' War.
After the signing of the Treaty of Limerick (1691) the soldiers of the Irish Army who left Ireland for France took part in what is known as the
Flight of the Wild Geese.
Subsequently, many made a living from working as mercenaries for continental armies, the most famous of whom was Patrick Sarsfield, who, having fallen mortally wounded at the Battle of Landen fighting for the French, said "If this was only for Ireland".
About a third of the infantry regiments of the French Royal Army prior to the French Revolution were recruited from outside France.
The largest single group were the twelve Swiss regiments (including the Swiss Guard).
Other units were German and one Irish Brigade (the "Wild Geese") had originally been made up of Irish volunteers.
By 1789 difficulties in obtaining genuinely Irish recruits had led to German and other foreigners making up the bulk of the rank and file.
The officers however continued to be drawn from long established Franco-Irish families.
During the reign of Louis XV there were also a Scottish, a Swedish, an Italian and a Walloon regiments recruited outside the borders of France.
The foreign infantry regiments comprised about 20,000 men in 1733, rising to 48,000 at the time of the Seven Years' War and being reduced in numbers thereafter.
During the American Revolution, King George III of England, hired German mercenary soldiers from some of the German principalities to supplement his Royal Army.
Although the German mercenaries came from a number of states, the majority came fro the German state of Hesse-Kassel.
This resulted in their American opponents referring to all of King George's German mercenaries as "Hessians", whether the Germans were actually from Hesse-Kassel or not.
The Spanish Army also made use of permanently established foreign regiments.
These comprised three Irish regiments one Italian and five Swiss.
In addition one regiment of the Royal Guard was recruited from Walloons.
The last of these foreign regiments was disbanded in 1815, following recruiting difficulties during the Napoleonic Wars.
One complication arising from the use of non-national troops occurred at the Battle of Bailén in 1808 when the "red Swiss" (so-called from their uniforms) of the invading French Army clashed bloodily with "blue Swiss" in the Spanish service.