IDF, Army Of Israel

The Romans were also noted for making use of auxiliary troops, non-Romans who served with the legions and filled roles that the traditional Roman military could not fill effectively, such as light skirmish troops and heavy cavalry. After their service in the army they were made citizens of Rome and then their children were citizens also. They were also given land and money to settle in Rome. In the Late Roman Empire, these auxiliary troops, along with foreign mercenaries, became the core of the Roman Army; moreover, by the time of the Late Roman Empire tribes such as the Visigoths were paid to serve as mercenaries.
• Medieval Europe
In the earliest Middle Ages it was the obligation of every noble to respond to the call to battle with his own equipment, archers, and infantry. This decentralized system was necessary due to the social order of the time, but could lead to motley forces with variable training, equipment and abilities. The more resources the noble had access to the better his troops would be.
The knights were drawn to battle by feudal and social obligation, and also by the prospect of profit and advancement. Those who performed well were likely to increase their landholdings and advance in the social hierarchy. The prospect of significant income from pillage, and ransoming prisoners was also important. For the mounted knight war could be a relatively low risk affair.