The Ottoman Empire janissaries were a mighty force - an infantry unit which was loyal to the Ottoman Empire. One would think they were Moslems, but in actual fact they were Christian slaves. They were the very best and so they formed the Ottoman Sultan's household military and bodyguards.
This force comprised of Christian slaves who were taken from their villages as young children - aged between seven and ten- and were brought up to owe their allegiance to the Ottoman ruler.
The Ottoman Empire janissaries, was a force formed in the 14th century and this force existed right up till the year 1826 when they were abolished by Sultan Mahmud II. Initially the Ottoman Empire Janissaries comprised of Greeks and Albanians. But this trend did not exist for too long because the requirement for soldiers was too high.
They began to include Serbs and Bosnians as well as people from other Bosnian countries, even from Russia and Ukraine, among this elite force. At first there was a system of recruitment, but later on this system was abolished and only volunteers were recruited for this elite army.
And volunteer they did because being among the Ottoman Empire Janissaries was actually a great way to eke out a living. These soldiers were rewarded handsomely for their endeavors. They were paid a really high wage and had extremely high standards of living. What was more, they were permitted to retire from the force as free men (and later on, even women were included).
They were regarded as slaves because they were a part of the Ottoman Sultan's property, but their elevated status made them actually covet the role. There was just one catch, they had to convert to Islam as proof of their allegiance to the Ottoman emperor and they did so willingly.
In fact, apart from this, quite a few measures were taken to ensure that the Ottoman Empire janissaries owed their loyalty to the Sultan and only the Sultan. For starters, they weren't allowed to take up any profession other than that of being a janissary. They were also required to abstain from sexual interactions initially. Later on, they could marry, but it was not mandatory for their offspring to join service.
Unfortunately, the Ottoman Empire Janissaries became more of a liability than an asset by the 1800s, so they were banished in the 1800s and finally a European style army was established to serve the Emperor.