The ancient Roman Empire was not always ruled by an all-powerful emperor, but power was split between prominent military leaders, ambitious politicians and a legislative Senate based on Greek political ideals. Once Augustus Caesar assumed the title and power of imperator or emperor, however, the hierarchy of power shifted towards a titular ruler who controlled both the military and the Senate, in as much as it were possible during those turbulent and violent times.
The emperor also held a more militaristic title of praetor, much as a modern president is also considered a commander-in-chief. Because the praetor was a constant target of political and military opportunists both foreign and domestic, an elite group of experienced soldiers was drafted to form a Praetorian Guard. The Guard's first and only loyalty was to whichever praetor held power at the time.
Under the protection of Praetorian Guard units, the emperor was free to walk through the halls of the Senate or the streets of Rome with little fear of assassination or violent confrontation. If the emperor wanted to visit a distant battle site, a large guard detail was automatically dispatched. Although individual Praetorian Guards were still considered soldiers in good stead, they were generally spared from front line combat duties while in the service of the praetor.
As the Roman Empire began to crumble, however, a number of members of the Praetorian Guard began to exert their own political muscle. Strict loyalty to the emperor became a secondary concern as guard members pursued their own political ambitions or fed the engine of political "machines" seeking the overthrow of the present empirical ruler. Corruption became rampant, which prompted at least one emperor, Constantine, to order it completely disbanded.
Members of the Praetorian Guard during the height of the Roman Empire could best be compared to the modern Swiss Guard which protects the Pope or the legendary French Musketeers who pledged their undying loyalty to the French king. The Praetorian Guard might perform routine protective duties in the emperor's private quarters one day, then quell a civil uprising or reinforce a besieged regular army outpost the next.