In 100 BC, Lucius Appuleius Saturninus was tribune and advocated several social reforms, among which was a bill that gave colonial lands to war veterans, a suggestion that was radical and displeasing to the patrician senate, which opposed the measures. Violence broke out and the Senate ordered Gaius Marius, as Consul for that year, to put down the revolt. Marius, although he was generally allied with the radicals, complied with the request and put down the revolt in the interest of public order. The political issue of land allocation for Rome's military veterans would return several times to haunt the state including 14 AD when an army in central Europe mutinied over the failure of the state to provide land plots for soldiers.
After the conclusion of the Social War, certain of Rome's eastern provinces became under threat of invasion and it was necessary to raise an army to counter the threat. The choice before the Senate was to put either Consul Marius or Consul Sulla in command of an army. There was already a fierce rivalry between the two, in part due to a competitive instinct amongst the two as successful generals, but more importantly distrust on Sulla's part that Marius held unhealthy ambitions. The Roman Republic was always on guard against any citizen gaining too much prominence, lest he seize power and restore Rome as a kingdom; thus a series of checks and balances existed, such as consuls having to be re-elected annually. Marius had already served five consulships and enjoyed widespread popularity. The senate made its decision and Sulla was given the job but a short time later the decision was reversed by the Assembly, and Marius placed in command. Already wary of Marius' prominence and previous five terms as consul, and (rightly) suspecting bribery in the securing of the position to command the army (Marius had promised to erase the debts of Publius Sulpicius Rufus), Sulla refused to acknowledge the validity of the Assembly's action.
Sulla left Rome and traveled to reach the army waiting in Nola, the army the Senate had asked him to lead against Mithridates. Sulla urged his legions to defy the Assembly's orders and accept him as their rightful leader. Sulla was successful and the legions stoned the representatives from the Assembly when they arrived, defying the state's orders. Sulla then commanded six legions to march with him to Rome. This was a momentous event, and was unforeseen by Marius, as no Roman army had ever marched upon Rome - it was forbidden by law and ancient tradition. Marius fled with no great loss of life and Sulla later disbanded his legions and re-established consular government, but the military had been shown to be able to be used as a political tool of individuals. It was a pattern to be repeated more famously later by Caesar.
During the First Triumvirate of Julius Caesar, Pompey and Crassus, each of the triumvires used military success to enhance their own political and public status. The incredibly wealthy consul Crassus, who had earlier displayed his wealth by entertaining the populace of Rome at a huge party with 10,000 tables, personally raised and funded six legions from his personal wealth. Whilst he did not at this time use them for marching on Rome in direct furtherance of his own career, his desperation to rival the military campaigns of Pompey that led to recognition in the public eye means that his motives are unlikely to have been entirely selfless. Rather, it was another step in the severance of the direct connection between state and troops that saw armies increasingly become tied to their generals' political careers.
When the triumvirate collapsed, Caesar crossed the Rubicon river and marched his armies upon Rome itself. This turning of an army loyal to its general against the state had occurred before under Sulla, but the circumstances were different this time: Sulla felt at least partly justified in his march on Rome by the alleged and probably real corruption of the political system by Marius, and by Marius' own quest for primacy as a political figure against a political backdrop that sought to prevent any person becoming too prominent. Caesar, on the other hand, marched his army against Rome purely for his own purposes. It is true that the political maneuverings of Pompey, which made it possible for Caesar to be prosecuted on his return to Rome, pushed Caesar into marching on Rome, but the fact that almost the entire senate fled alongside Pompey shows that Caesar's actions were at least perceived to be an act against the state itself rather than the person of Pompey: Caesar's power base was built almost exclusively on the loyalty of the soldiers who had served under him campaigning several years. Unlike Sulla, Caesar also failed to revert power to the state when the threat of Pompey had been removed, not just keeping his position as de facto ruler of the state but, immediately on his return from defeating Pompey, naming his grand-nephew Gaius Octavius (Octavian) as the heir to his title, a wholly unconstitutional act. In everything but name, the army had placed the first Emperor on the throne of Rome.