For the military historian, the main feature of what is usually called the modern period is that it is dominated by the professional soldier, hired at home or abroad. In the early part of the period, troops were recruited abroad to serve for a limited period only: these were the mercenaries or ‘hired troops’ in the strict sense, of whom the purest example were the Lansquenets, troops completely unattached to any one state or government, who sold their services to the highest bidder on terms mutually agreed and laid down in a deed of engagement. Mercenaries, however, came to be replaced by troops of home origin, engaged by the state for an indefinite period of time and organized into regiments or companies which were permanent sections of the military forces; that is, by a standing or national professional army. In practice, such standing armies also comprised a varying number of foreigners, either grouped into separate regiments or spread among the various sections of the army. Naturally, the process of transition from the one method of raising an army to the other differed from country to country, but in general it can be said to have taken place in the first half of the seventeenth century. This was closely bound up with the development of the modern state and with the evolution of absolute monarchy, the most powerful weapon of which the standing army proved to be.

The use of hired troops can be traced far back into the Middle Ages and predominated from before the beginning of the sixteenth century. But, obviously, governments forced to take hired troops into their service could not close their eyes to the danger of entrusting their own and their subjects' interests to foreign adventurers, nor to the thieving, plundering, extortion and other outrages which made these bands the terror of friend and foe alike—not only in the field, but also after they had been disbanded.