History explains why cardinals under 80 years of age have the exclusive right to choose the leader of a billion-plus Roman Catholics, and why they vow to shun all outside influence, whether from clerics, parishioners or — most importantly — political powerbrokers. So important is keeping the cardinals sheltered that Vatican workers took an oath yesterday never to reveal anything they may learn about the workings of the conclave.

As the church grew from a small sect into a powerful force, both spiritually and temporally, the pope across 11 centuries was a monarch who ruled central Italy. Political feuding for his throne sometimes became unbearable.

Nineteen elections lasted more than a month and several dragged on for many months, even years. Other successions descended into violence.

Consider the reign of Pope Sergius III in the 10th century.

His faction seized the papacy through armed force, and he had his imprisoned predecessor, Pope Leo V, strangled to death.

Chroniclers of the age claimed that Sergius had a son with a 15-year-old girl — who was later elected Pope John XII by the nobles who had backed Sergius. John himself became a notorious debaucher who was deposed, struck back and deposed his successor, and supposedly was in bed with a married woman when he died.

Then there was Alexander VI, who won the papacy just before the Protestant Reformation by bribing cardinals and promising lucrative jobs. His subsequent reign was "marked by nepotism, greed and unbridled sensuality," the Rev. Richard McBrien writes in "Lives of the Popes."

Thanks to Alexander, the cardinals who elected his successor included his illegitimate son, appointed a cardinal at age 18, and the brother of one of the pope's mistresses.

Perhaps the most bizarre succession involved the "cadaver synod" of 897.

So much did Pope Stephen VI hate his deceased predecessor, Pope Formosus, that Stephen had his minions dig up Formosus' corpse. The new pope then held a mock trial for the old one, stripped the corpse of its vestments, cut off the two fingers that bestowed papal blessings and threw the body into the Tiber.

The over-the-top display did Stephen no good. His enemies rebelled, imprisoned him and strangled him to death.

Such outrageous behavior was not how it all began.

St. Peter, the first pope — though historians say he was not called by that title in his lifetime — named his successors.

And on many occasions in the first Christian millennium, popes were chosen by the clergy and laity of Rome — or by the clergy with assent from the laity.

By 1059, the church decided to try and prevent electoral pressure from mobs, Roman nobles or European emperors and gave election powers only to Rome's ranking clerics, the cardinals. Since 1378, only fellow cardinals have been elected pope.

Still, worldly politics were hard to stamp out. The conclave concept was inspired by a particularly messy 13th-century election that required 33 months because cardinals represented competing monarchs.

The angry mayor and townspeople of Viterbo, north of Rome, eventually locked the cardinals in a papal palace, reduced their food and ripped off part of the roof to expose them to the elements, all to force a decision.

The cardinals' monopoly on power has been broken only once, to end the disastrous Great Schism (1378-1417) when two and sometimes three rivals claimed to be pope. The Council of Constance assigned 30 prelates representing the major powers of Europe to join the 24 cardinals in choosing the pope.

The papacy's political problem had roots in the eighth century, when Pope Stephen II allied with the Franks against Germanic and Byzantine monarchs and established the Papal States — making the popes absolute rulers of a sovereign, secular realm.

That regime fell to Italian troops in 1870, and a 1929 treaty with Italian dictator Benito Mussolini established its remnant, Vatican City.

But during the centuries when the papacy held political and financial power as well as religious authority, European monarchs jockeyed for influence over the church; eventually they even gained veto powers over papal candidates.

As recently as the 1903 conclave, a cardinal from Krakow, Poland — the same post John Paul II later filled — declared that the Austrian emperor was exercising his veto right against the front-runner. He and the other cardinals were furious, and claimed the veto was invalid.

The objectionable candidate then gained a bit of momentum, but in the end the conclave elected Pius X. Still, the cardinals' anger signaled a permanent change in the papacy.

While the church regarded the loss of the Papal States as a disaster, the defeat actually had liberated the popes to become strictly moral and spiritual leaders again.