"During the sixteenth and seventeenth centuries the most common was that the various armies in conflict were plurinational. Sometimes the plurinationality of the army derived from the fact that the monarch was at the same time from several different states; although it was normal for it to be produced by the presence of foreign troops, what we would call mercenaries today, understanding as such those that served a lord or state that was not his natural one.
The recourse to foreign troops was nothing new and there were extensive examples in previous centuries. But now the expansion of the armies, especially in regard to a greater presence of disciplined infantry, generated an immense demand for professionals that the "internal market" of the main states in conflict could not meet. By the 30s of the 16th century, it was usual for states to need twice as many men as they had in the past.
We are at a time when in most states permanent armies, in a modern sense of definition, are relatively small or directly non-existent.
The alternatives presented to the states to undertake a great campaign did not leave much room. The conscription systems were barely developed and the traditional militias had limited effectiveness; useful in the best case for the defense of the territory, but of little use in an offensive campaign. In addition to a limited willingness of the militias to operate outside their territory, the various legal and political restrictions in most states that caused a monarch to "negotiate" permits for their recruitment and use.
There was the option of seeking volunteers from among the subjects, of course, but generally they were not in sufficient numbers or with the necessary preparation for an immediate campaign. In the end, the quickest solution was to resort to "professionals" of war. Professionals that abounded in certain areas such as Germany, Switzerland or the Balkans; as well as those that were "unemployed" in certain countries when demobilization arrived when a period of peace (or bankruptcy) came. In addition to populated but relatively economically backward regions, other exporting regions were those that suffered turbulence of a political or religious nature, leading to exiles.
It would not be until the second half of the seventeenth century when, by the hand of fiscal and administrative reforms, the great states could go reforming their armies, and go on installing recruitment systems for the maintenance of some armies…"
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