IDF, Army Of Israel

As central governments grew in power, a return to the citizen armies of the classical period also began, as central levies of the peasantry began to be the central recruiting tool. England was one of the most centralized states in the Middle Ages, and the armies that fought in the Hundred Years' War were, predominantly, composed of paid professionals.
In theory, every Englishman had an obligation to serve for forty days. Forty days was not long enough for a campaign, especially one on the continent.
Thus the scutage was introduced, whereby most Englishmen paid to escape their service and this money was used to create a permanent army. However, almost all high medieval armies in Europe were composed of a great deal of paid core troops, and there was a large mercenary market in Europe from at least the early 12th century.
As the Middle Ages progressed in Italy, Italian cities began to rely mostly on mercenaries to do their fighting rather than the militias that had dominated the early and high medieval period in this region. These would be groups of career soldiers who would be paid a set rate. Mercenaries tended to be effective soldiers, especially in combination with standing forces, but in Italy they came to dominate the armies of the city states. This made them considerably less reliable than a standing army. Mercenary-on-mercenary warfare in Italy also led to relatively bloodless campaigns which relied as much on maneuver as on battles.