I don't think anybody seriously disputes that Sulla had justified reasons to object to the crisis occurring at Rome in 88. The question, going back to Mommsen, has always been a) whether the use of force was a reasonable way to resolve the crisis and b) whether Sulla's stated justification was the reason why his troops joined him in civil war.

The most recent treatment of the crisis of 88 is Morstein-Marx's article "Consular Appeals to the Army in 88 and 87: The Locus of Legitimacy in Late-Republican Rome," published in 2011. Morstein-Marx makes the important distinction between legality and legitimacy. What Sulla did in 88 was, unquestionably, not legal, but even Cicero, as outright civil war threatened in March of 49, could deny that Sulla, Cinna, and Marius had begun their conflicts rightly and "perhaps even justly" (immo iure fortasse), and that the criticism of their decisions was due to how they acted after securing victory, slaughtering their enemies and acting entirely against the notions of Roman government (Att., 9.10.3). The ancient authors likewise tend to depict Sulla as having been undeniably treated unjustly, and only turning to outright tyranny after eliminating his opponents. Even Appian, who is probably the ancient source most sympathetic to the pre-Social War Italians, and has a very unfavorable view of Sulla's heavy handed treatment of Sulpicius' proposed law, presents this ambivalent view of Sulla's behavior.

So what happened in 88? Early in the year, the tribune P. Sulpicius promulgated a law that would distribute the newly-enfranchised Italians into all the tribes, rather than being restricted as they were at that time to only ten (or eight, there's some dispute). It's important to recognize that only a few years later, this came to be, and in the post-Sullan period the Italians belonged to any of the thirty-five tribes. So clearly public opinion eventually was behind Sulpicius' motion, and the sources give us reason to believe that already in 88 the restriction of the Italians to certain tribes was pretty widely unpopular. Sulpicius increased his political clout by two means. First, he employed violence, apparently to prevent the consuls (Pompeius and Sulla) from coming out into public to speak against him, driving both consuls from the city after their abortive attempt to stop him from holding a vote on his law by declaring a halt to public business--in the fighting, Pompeius' son was even killed. Second, with the consuls out of the way, Sulpicius promulgated a law that would transfer leadership of the war against Mithridates from Sulla, who had been assigned it as his province, to Marius, who was at that time a private citizen. The timeline on that one is a little unclear. The sources claim that Sulpicius and Marius had made a secret pact before Sulpicius' election, but that seems unlikely in the extreme. More likely, Sulpicius saw the opportunity to gain an influential political ally after having (kind of accidentally) driven Marius' chief rival from the city.

Sulla's justification, the sources unanimously declare, was this latter outrage. Up until now the constitutional crisis of 88 had been restricted to a dispute between a tribune and the consuls. While concerning, it wasn't unprecedented. When this kind of thing had happened before a state of emergency had been declared and the consuls empowered to put down whoever had been declared a public enemy (hostis). Indeed, the procedure of declaring a public emergency had first arisen in 122 to empower the consul Opimius to put down the threatened sedition of the tribune C. Gracchus and his consular ally Fulvius Flaccus, who had armed his supporters with weapons and armor. However, while commands had been transferred from one magistrate to another, and while that sometimes made a lot of egos very unhappy, in the past such reassignments took the form of a senior magistrate being granted higher command in the assigned theater to lower-ranking subordinates. That's how Marius had been assigned command against Jugurtha--in 107, the Roman people voted in the assembly to dispatch the consul Marius as the overall theater commander in Africa, taking over from the lower-ranking proconsul who was already conducting the war there. In 88, Sulla, a sitting consul, had already been assigned by the usual procedures to an ongoing war. Sulpicius' law deprived the consul of that privilege, and replaced him with a private citizen, i.e. someone who didn't even have any rank at all. Diodorus 37.29.2 outright calls it and illegal law, although I think it's kind of a grey area, and that if anything Sulpicius was probably well within his rights to propose the law, even if its actual contents were pretty objectionable. In the Republic, the assembly was conceived of as having the ultimate say over the assignments of the magistrates, who were thought of as existing to carry out their commands, and there are other examples of the people's intervention in the ordinary affairs of provincial magistrates (consider that the Roman people were the ones who voted on whether to declare war). It should also be noted that Marius did not have the best track record when it came to taking over from other commanders. Both times that he had been assigned to an open theater of war, in the Jugurthine War and in the war against the Teutones and Cimbri, he had not only completely taken over the narrative of the war, assigning all the successes and even the entire conduct of the war to himself (even though in particular by the time he arrived in Africa the tides of war were probably already changing) at the expense of the previous commanders and his own subordinates (one of whom had been Sulla himself), but had even replaced the troops fighting the war, enlisting an entirely new army.

When Sulla arrived at Capua, where the army that had been conscripted to fight the war against Mithridates was encamped, he held a military contio, basically the military form of the public speeches given in the forum by magistrates in civilian life. I mention specifically that it was a military contio because, as Morstein-Marx has underlined, the events of 88 demonstrate how a) the personal considerations of magistrates and the public ideology of Roman political values were very closely intertwined and should be evaluated as different parts of a coherent whole, and b) the intersection between the ideological position of the military and that of the Roman people. Sulla's contio, according to Appian, stressed the injustice that Sulpicius committed against him, and by extension against the office of the consul, a point which will become apparent in a bit, because I'm going to walk you through the similar incident with Cinna the next year, which is better documented. Appian claims that Sulla's troops were eager for plunder. Mommsen interpreted that as meaning that the troops desired to plunder Rome itself, but at this stage there was no talk of storming the city, and more recent scholarship points out that plunder was always the overriding concern for all Roman soldiers, and was typically one of the most important considerations for the assembly when it voted for war. Morstein-Marx more plausibly reconstructs the soldiers' concern as fear that Marius would replace them and deprive them of their opportunity to benefit from the war. And indeed, it probably looked like that's exactly what was about to happen to the troops on the ground. Plutarch tells us that Marius raised a new crop of military tribunes and sent them to the army at Capua. These tribunes would have replaced the ones already assigned to Sulla. According to Plutarch, when the new tribunes arrived the soldiers, presumably interpreting this as the first step in conscripting an entirely new army (Plutarch says that Marius wanted the new tribunes to bring the army to him at Rome, but that seems doubtful, given that the army wouldn't have embarked from Rome and the replacement of the tribunes is an odd gesture), stoned Marius' tribunes to death.

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