The Jewish Elite and the Romans had a shared problem.
The way in which armies took over other nations was appallingly simple in its initial execution, and the pattern wasn't merely Roman. Invade, conquer militarily, kill the existing leader publicly, and then install your own government.
Things were pretty uneasy in Judea, however. The Jewish people not only smarted under Roman occupation in the way that other nations had - never mind the taxes - for example, a Roman centurion could compel anyone to carry his personal burdens for a mile - but they had a lot of serious religious and cultural objections to the way things were done.
To make an incredibly long story short, what transpired in the end was a very uneasy balancing act in which the Jews had a king (Herod) but also an Emperor. They had the coin of the realm, but also a private temple currency (and therefore, a thriving business in money-changing). Though there were occasional spats and insurrections, and the region was a powder keg, things were for the moment under some sort of control.
The people were groaning under a dual burden, as a result. The practical upshot was that they had obligations to two uneasily balanced factions: they had religious legal obligations and Roman legal obligations - they had taxes to pay to Caesar (marked up by tax collectors) and obligations to pay to the religious authorities, which now came with the double insult of a "tax" on top of these obligations by virtue of the profit margin of the money changers.
With all due respect to my Jewish brethren and not wishing to offend, on the other side of things, (from a Christian perspective) there were differing camps of Pharisees and Sadducees and some of them were trying to codify Jewish obligations in a way that had you, for example - counting the steps you took on the Sabbath to avoid taking too many. Advanced Dungeons And Dragons, second edition rules seemed simple and straightforward by comparison.
In the midst of this came a laborer from a backwater whose audience kept increasing. And he was interfering with this finely tuned balance of power. In direct defiance of the religious leaders of the day, he had his followers ignore various rules they imposed. He field-stripped down the expectations of anyone listening to him to "be good to other people, even your enemies". When referring to God he used a child's term- the Aramaic version of "Daddy" - describing a personal, reachable God that needed no religious intercessor.
Of course, it isn't as if he wasn't causing ructions with the Roman power, either. He taught his followers that if a Centurion compelled you to take a burden a mile, to carry it for another mile. This meant that a Centurion ran the risk of being seen to have someone carry a burden beyond the limit imposed by law. "But he carried it another mile even though I told him not to" might or might not be believed. It was nonviolent resistance at its core.
The Jewish elite formulated a plan to get rid of him.
He was seized in a Garden, while praying, late at night. Far from the crowds that followed him by day - the only people with him at the time were his closest compatriots, and they were sleeping. Having paid a member of his inner circle thirty pieces of silver to point him out to the armed multitude (a mob?) who would seize him - they brought him in dead of night to the palace of the high priest, Caiaphas. After a trial punctuated by false witnesses (Jesus once said if you destroyed the Temple, he would rebuild it in three days. This was restated in the "court" that he was the one able to destroy the Temple, a threat), they cut to the chase and directly asked if he was the Son of God. His answer seemed proof of their charge - the charge was blasphemy, and the penalty was death.
There was only one minor problem.
They weren't actually able to carry out such a sentence.
When they delivered Jesus to the Roman governor, Pontius Pilate, the charge was sedition. If Jesus had aspirations to be King of the Jews, this would be in direct defiance of the rule of the Roman Emperor. The Gospel of Luke carries it further, having them accuse Jesus of telling the Jews not to pay tribute to Caesar, even though his answer on that question is even today a colloquial expression - "render unto Caesar that which is Caesar's."
The place was a powder keg. In the back of their mind was the events of 4 BC following the execution of some Jews removing a Roman Eagle temple decoration - resulting in the crucifixion of two thousand people after a city wide riot. Luke mentions the death of some Galileans beaten to death for amassing and starting an insurrection over the use of sacred treasury funds to redirect a waterway. There are two conflicting sides to this: though the Romans don't want to start a riot by killing someone a week before was greeted with adoration and palm fronds, they also don't want a baying mob requiring the redirection of a Legion, again. A typical politician, Pilate tries to extricate himself from a sticky decision.
Though Pilate clearly sees no problem or fault in Jesus, the story diverges, here - in Matthew's gospel, Pilate tries to extricate himself of the affair and sets up a choice for the people - to release either Jesus or a notorious murderer by the name of Barabbas. In that version of the story they choose Barabbas, which Christians since see as a metaphor and allegory for Jesus accepting the punishment as an innocent man for the sins of others, granting them freedom from the punishment which they are due. In Luke's Gospel, on finding out that Jesus is Galilean, decides that it's Herod's jurisdiction and not his. Herod is excited, hoping for a personal audience and a miracle or two, but Jesus answers neither man. In Luke's gospel Jesus is flogged in lieu of crucifixion to appease the baying mob (in an attempt to save his life) - in Matthew's Jesus is scourged simply as a prelude to the rest of the execution.
The "scourge" in question, (Greek: μαστιξ) as it's commonly understood was made of leather and tipped or embedded with metal and bone, designed to shred tissue as opposed to simply cutting it. One of the few benefits of watching the Mel Gibson torture porn film The Passion of the Christ is the instructive moment in which a lictor strikes a wooden object with the whip to intimidate Jesus, and it sticks to it - requiring him to pull the weapon back out, ripping it free. Another piece of trivia is that the actor playing Jesus in this scene was actually struck twice with said weapon (by accident), and the second blow served as the model for the phenomenal makeup job showing the result of the use of a scourge.
Cicero says of this sort of thing "pro mortuo sublatus brevi postea mortuus" (taken away for a dead man, shortly after he was dead.) The following morning, as Jesus is led away to carry his cross (most certainly the crossbeam or "patibulum") he is so damaged and in hypovolemic shock that he drops it multiple times. According to three of our (the synoptic) Gospels, the centurion in charge of the execution, fearing that his prisoner wouldn't make it to the sentence being carried out, orders another (Simon of Cyrene) to carry it for him.
The standard movie image of a relatively healthy man struggling under the weight of an entire cross is unrealistic. Regardless of the better strength and stamina of a laborer in a time before cars, healthy and unmolested actors portraying Jesus, as well as experiements run with volunteers tracing the actual via dolorosa with a model of the classic Stations of the Cross portrayal show it to be near impossible. It could certainly be unrealistic to expect someone dealing with the aftermath of scourging to even make it part way. Nevertheless, it's been used as a metaphor for Jesus taking on the weight of the sin of the world, and has been seen as a reason why John does not include Simon of Cyrene in his narrative, wishing to emphasize Jesus taking this weight on his own shoulders.
We have no idea what shape the cross took, whether it was the shape seen in most churches, a Tau cross, or what have you. Or how many nails were used or even where they were inserted. The Bible is unclear, because it's irrelevant. Whether the nails were put through the palms of the hands or the wrists, or there was one nail through both feet or a nail through each ankle, is also unclear, and irrelevant.
He was most certainly naked at the time. The Last Tempation of Christ got that right. Part of the penalty was the humiliation of being affixed and displayed naked, having to defecate and urinate in open view as you died. (The reason why the classical depiction of the crucifixion had Jesus in a loincloth was not modesty, but that in order to do it realistically one would have to show that he'd been circumcised, and that would have opened a can of worms in terms of medieval relationships with the Jewish people.)
What is known is that, living tree or tau cross, Jesus was affixed to it with nails and left to die, with the inscription above "Iesus Nazarenus, Rex Iudaeorum" (Jesus the Nazarene, King of the Jews). And at this point the uneasy balance of earthly powers clashed. The Jewish leaders demanded this to be changed to read "he SAID he was the King of the Jews". Pilate ignores this. He might have carried out this unfair sentence to calm a baying mob, but there's limits to how much he will be dictated to. The charge is not blasphemy or some internal Jewish squabble, but sedition. However, it's not "seditio" - it's there for a reason. The inscription sends a message to everyone concerned. Don't mess with Rome. Message sent, message received. The elite don't push it and retreat angrily.
Jesus' followers watched in horror and dismay. It wasn't supposed to end like this. They were looking for a messiah. They were looking to see the promises fulfilled. They were looking for Olam Ba Ha: an end to war, Judaism as the only religion, no murder or robbery, the end of all struggle, united in military victory. An Israeli government that unites the entire world. Seeing somebody in agony, a close personal friend and mentor - was horrifying on one level, but even more in the sense that it shattered a dream.
The fact that Jesus said this would happen, that the victory would come in a way they wouldn't expect, was lost on them. That he rode in on a donkey, not a horse - the horse was reserved for war and he did not ride in on one - also lost. He alluded to Moses raising the serpent in the desert, so that those who would see it would have the venom in their veins negated by viewing it - the death on the cross would give them a path to removing their own taints and sins.
Even so, there were those who expected him to rally back, like Hulk Hogan suddenly shrugging off the attacks of his opponent, or the cliche of the action hero stirring to his feet after a near-death beatdown. There was hope in some people saying, "come down off that cross and save yourself." Others simply mocked. There were expectations of a messiah and he didn't fulfil them, so his was the death of a troublemaker. "Rebuild the temple now, if you can."
He died between two men. Depending on the way you translate the term, they were either thieves, or they were insurgents. Either way one taunted him saying "save yourself, and save us as well". The other, either believing that Jesus was Lord or simply offering comfort and sympathy at such a time, told the first man to shut up, that they deserved their penalty and should accept and take the blame for their own faults, instead of being angry at the blameless. In this, Christianity expresses two attitudes to our relationship with God - contrasting the one that makes demands of God and demands things done his way, and the other, who asks for repentance.
A combination of the cramping of the chest (causing asphyxiation), hypovolemic shock, and other factors result in Jesus dying, near delirious and asking for water. After final words (either "It is accomplished" or "why have you abandoned me?") he dies, not needing his legs broken to hasten his death before sundown and the Sabbath. A centurion pierces his side to see if he's dead for sure, and verifies the death with this blow.
But the most important part of the story occurs afterwards. The biggest imperial power the world has ever seen, combined with the top echelons of the religious elites, have conspired to do just about the worst one can do to a human being. Separated from his friends who are too scared to show their faces (one of whom actually denies knowing him), tortured mercilessly with open handed slaps, a crown of thorns, flagellation and being nailed naked to a stake - he died in agony. Shamed, disgraced - and for all intents and purposes suppressed forever.
And yet, strangely enough, three days later he was walking around, eventually eating fish with his friends and telling them it was now their turn to carry on the ministry. Restored in body, restored in soul, with the faith of his friends renewed. The net result is that everything they tried to achieve with the execution simply failed. Failed catastrophically in every single way. Wounds? Healed. Death? Shrugged off as a nuisance. His message? Available in every bookstore today. It represents the complete unimportance of the physical world compared to the spiritual, and the absolute and eventual failure of worldly power.
A theme carried on by Shelley, later:
"My name is Ozymandias, king of kings:
Look on my works, ye Mighty, and despair!"
Nothing beside remains. Round the decay
Of that colossal wreck, boundless and bare
The lone and level sands stretch far away.
The roads that carried the centurions and oppressors of Roman power would soon be used to carry a Gospel. The language they imposed on others would be a lingua franca which would transmit ideas across time. The instrument of torture and destruction, instead of an object of shame and humiliation, would come to be used as a motif representing hope, and new life. Regardless of the religious meanings of the event, and out of respect for the differing belief systems here - even from a secular perspective it represents the triumph of his ministry.
Speaking of secular triumphs - as for the empire the Jews were groaning under? Finished. When we think of Rome we think of aging ruins and pontiffs, not the Eagle. But even before the barbarians beat down the gate, Christianity ate at it from within so that it was no longer the same conquering power it was when it arrived in Judaea. The religious elites? Scattered to the four winds.
Neither power that Jesus opposed remains.
In the end, he won.