The Janissaries and their various guises over time are an excellent example of the potential for power to cause corruption. Formed in the late 1300's, the Janissaries were an elite army belonging to the head of the Ottoman empire (the bey or sultan depending on the time period). They were responsible for guarding him and his family as well as carrying out his military missions. The Janissaries' position was somewhere between what we think of today as a slave and a military conscript. As young men they were taken from Christian families and forcibly converted to Islam. This was intended to ensure that they were loyal only to the ruler, and hence the empire. Like slaves, they literally were considered property of the sultan and like slaves, they usually were kept from contact with their families and even much of free society. However, the high social status of Janissaries, the fact that they received a wage, their standard of living and the fact that they could retire from the Janissary corp as free men are all more similar to the lives of military men (and women) who have been drafted. Since becoming a Janissary was a way for a poor Christian boy to become wealthy and powerful, some saw being conscripted much like we would see winning big in the lottery.
A number of additional measures were taken to insure that Janissaries' loyalty did not become divided. They were not allowed to have a job beyond their military duty. Initially Janissaries were required to be celibate. Later, they were allowed to marry, but their sons could themselves join the Janissaries, so that nepotism could not influence the balance of power within the corp. As time passed, the Janissaries gained power - both through their effectiveness as a practically unbeatable military force and by rioting for better pay and conditions. They gained the right to have their sons join the corp. Janissaries also started getting into trades and focusing on their businesses, not their military duties. Eventually, they gathered so much power that they were a law unto themselves. If they did not like what the Sultan was doing, they would revolt. The Janissaries were also resistant to change and did not attempt to adapt to modern military methods. By the 1800's they had become an expensive, political liability to the Sultan. In 1826, Sultan Mahmud 2 finally had to start a new European-style army to maintain the empire's military power. When the Janissaries revolted due to their replacement, their barracks were destroyed and those Janissaries who were not killed were banished.
Oh, and the term janissary, also spelled janizary, - it comes from the Turkish yan cheri. It means new army, as opposed to the previous Ottoman army which consisted of volunteers who were more interested in potential loot and glory than meeting any particular strategic goals.
More Janissary pictures at Art.com