During his stint in the military, Tiberius Sempronius Gracchus (168-133 BC) fought under cousin Scipio Aemilianus in the Third Punic War (149-146 BC), where he was reportedly the first man over the wall during the siege of Carthage. Later he served as quaestor in Spain under G. Hostilius Mancinus. There, Tiberius was instrumental in negotiating the peace treaty with the Numantians (a Celtiberian tribe resisting Roman domination), which probably saved the routed Roman army from destruction. This treaty would eventually be rejected by the Senate, and Mancinus would be put on trial for his failures as a general and turned over to the enemy. The officers serving under him and involved with the treaty were only spared a similar fate because the people credited Tiberius with saving the lives of thousands of Roman soliders.
The political career of Tiberius Gracchus began in earnest in 133 BC when he was elected a Tribune of the People. As tribune, he decided to take on the issue of land reform as his personal mission. This was perhaps the biggest social problem of the day: Rome was home to an ever growing mob of poor, landless citizens. There were several reasons for this: sometimes wealthy Senators would hire goons to strong-arm small farmers off their land. Also, a Roman father would usually divide his landholdings among his sons so that over the generations his descendants might not have large enough parcels to support their families. Most of these displaced farmers would move to the capital city of Rome where shanty towns were built around the city center: they would survive on handouts (distributed by the wealthy on occasions such as triumphs and religious festivals), watch free games in the arena, and maybe sell their vote. Some of these masses no doubt turned to a life of crime whereas others mulled around just waiting for a good opportunity to erupt into some mob violence. In a lot of ways, life in Ancient Rome was comparable to the conditions existing today in overcrowded, third world cities like Mumbai.
Tiberius was particularly concerned about landless veterans who, ideally, should have been given a plot of land at the end of their tour of duty. The growing number of landless citizens also placed a burden on the army since at the time only members of landholding families could serve in the military.
In order to combat the problem, Gracchus wanted to redistribute public land. This was land the Roman state had conquered during its expansionist wars, and at the time large swaths of it were controlled by Senators and other members of the wealthy elite. Tiberius proposed a law (known as the lex sempronia agraria) which would enforce pre-existing limits on how much public land one man could control, with the excess being confiscated and parcelled out to landless veterans. Unsurprisingly, this measure met with a lot of resistance from the Senate and ruling class since these were the same people who were bogarting all the land! For example, Plutarch says of Publius Cornelius Scipio Nasica (c.183-132 BC, Nasica = "pointed nose"), Gracchus' biggest opponent in the Senate, that he "surrendered completely to his hatred of Tiberius. For he was a very large holder of public land, and bitterly resented his being forced to give it up."
Acts as Tribune
Tiberius made further enemies through the unorthodox tactics he used to try to push his agenda through. He realized there was no use in trying to get the new law passed by the Senate, so he put it up before the assembly where citizens could vote on it themselves. To block this move, the optimates (the party representing the elite) got his fellow tribune Marcus Octavius to veto the proposed vote. As representatives of Rome's common citizens, tribunes of the people were sacrosanct and laying hands on one or interfering with his official actions was a capital offense. Thus, because no one could mess with him, a tribune could essentially block any government action in the city of Rome with his veto so long as he was physically present. Tiberius didn't appreciate this "tribune of the people" trying to stop the people from voting on his land reform law so he convened the assembly to vote on removing Octavius from office. When Octavius tried to veto this vote as well, Tiberius had him forceably removed from the meeting place and replaced him with one of his supporters named Mucius. In this incident, you can argue that Octavius was acting counter to the will of the people, but Tiberius' actions were pretty clearly unconstitutional.
Tiberius also started playing hardball to get his proposed law passed: he began using his own veto power to block all the Senate's daily business until they reluctantly agreed to enact the lex sempronia. Plutarch tells us that his obstructionist measures included sealing the Temple of Saturn so that magistrates could not access the state treasuries which were held inside. Tiberius set up a commission to carry out the law composed of himself, his father-in-law Appius Claudius Pulcher (who was also the "first man of the Senate"), and his younger brother Gaius who was away at the time serving under Scipio Aemilianus in Spain.
But this was not the end of the conflict. Although the Senate had passed the law, they underfunded the appointed commission so that it was essentially powerless. Then, at the end of the year 133 BC, the King of Pergamum died and named the people of Rome as his heir. Tiberius decided to again by-pass the Senate and to bring bills before the Assembly which would determine how the estate would be administrated: specifically, he proposed that the money be allocated to funding the land reform. His enemies characterised this as an abuse of power, and they spread rumors that Tiberius had monarchical aspirations.
The optimates wanted to get rid of Gracchus, perhaps putting him on trial for daring to violate the person of M. Octavius, but they would have to wait until he was out of office to do this. This danger probably played an important part in motivating Gracchus to run for re-election for the next year. By doing so, he was once again acting illegally given that the Roman constitution specified that several years had to pass before an incumbent could run again for one of these annual offices (Term limits, baby!).
Violence broke out on election day (things had already gotten pretty hairy when M. Octavius was forced out of office). When Tiberius heard of a possible attempt on his life, his most loyal supporters armed themselves and gathered around him. Plutarch tells us that Tiberius put his hand to his head to indicate that he was in danger, but his opponents interpretted this as him asking for a crown. >Gasp!< When word reached the Senate, a group of Senators set off themselves to put down this coup. They made their way through the crowd as those people who were not die hard members of Gracchus' camp would have made way out of respect for the dignity of their office. We're told the Senators and their men then broke apart benches and used the chair legs to club Tiberius to death together with 300 of his supporters. Afterwards they chucked his body into the Tiber.
When things settled down, the Senate realized that things had gotten out of control and that Gracchus' assassination had stirred up much resentment among the common people. Therefore, as a concession, they took steps towards carrying out the lex sempronia and redistributing land. Meanwhile, Tiberius' most outspoken opponent, Nasica, was sent to Asia Minor on some pretext (even though he was the pontifex maximus) out of fear for his life. He died in Pergamum the next year under unknown circumstances.
Images: Roman denarius minted in 134 BC found on www258.pair.com; Glicee lithograph of T. Gracchus sealing the Temple of Saturn found on amazon.com.