I think you're a bit confused: Roman consuls were generals, more or less by definition. Until about 80 BC the consuls' primary duties were as military commanders. This was their original function, and Polybius in Book 6 treats military command as effectively their only function. It's why there were two consuls--so that, in the very early days, there was one on campaign while another could be on garrison duty at home--and it's where their powers as officials come from. Consuls were originally the only magistrates with imperium, the authority invested in them by the people as electorate to command armies and to have coercive power over citizen-soldiers, and until the institution of the urban and peregrine praetors the Romans didn't have any elected magistrates whose primary function was to serve as civil administrators. Even then, the praetors were expected to serve as military commanders when needed--even the urban praetor's most important civil function was to publish the praetor's edict at the beginning of the year, after which he was free to go on campaign if ordered by the senate or the assembly. Similarly, the consuls were typically only in the city for a couple of months at most until the first century. Their primary civic duty was to set the order of speaking in the senate for the year, to prepare the people for military campaign, and to preside over the assembly that elected their successors. After that was done they headed out on campaign. Even after Sulla, when the highest magistrates were expected to stay in the city (or at least Italy) for their year, provincial command was inherent to the office. It's just that for the consuls and the two highest praetors it was delayed for a year--but note that the majority of the praetors were posted to the provinces, not the city!

Until rather late into the imperial period, Rome had no professional military class, neither of soldiers nor of officers. Even after the end of conscription in Italy under the emperor Tiberius it took some time for the army to become fully professionalized (and even then we can raise questions regarding "ethnic" membership in the army and what that means exactly). Throughout the Republic, the army's officers were drawn from experienced soldiers, or they were appointed by the commanders. Hence the big stink reported to us in Book 42 of Livy, when the military tribunes enrolled centurions during a levy not according to seniority from men who had already been conscripted as centurions in previous campaigns, but by choosing those they saw fit. According to Polybius, male citizens were expected to serve for sixteen years before the age of forty-six if they were enrolled in the centuries of infantry, and ten if they were enrolled as cavalry. Polybius also says that in times of crisis men could be required to serve as much as twenty years. He probably means during the Hannibalic War, but throughout the early and middle second century Rosenstein has persuasively argued that the Romans were hard-pressed to find enough manpower to fuel long-term wars in Spain and in other meatgrinder theaters like Sardinia. Excessively long periods of service appear during this period in Livy, as do large numbers of volunteers, indicating a pressing need for more bodies that was not being easily filled. We can also question whether these requirements were really regularly met. Rosenstein posits are growing number of draft-dodgers in the period leading up to 133. And it's long been suspected that the ten years of military service that Polybius says was required before one could stand for elected office (remember that political hopefuls are uniformly those with the resources to be enrolled in the centuries of cavalry) was, no later than the last quarter of the second century, not typically observed--a recent dissertation by Noah Segal seems to confirm this suspicion. Nonetheless, the expectation at least in the middle Republic was that young men, regardless of class, would have at least some military experience before entering their mature lives.

Polybius, as I said, says that ten years of military experience were required before one could stand for elected office. Cases where we know for a fact that that happened are relatively few, and they tend to be young men whom the sources point out for delaying their political careers by serving longer than they were expected. C. Gracchus is a good example. Plutarch tells us that when he became quaestor he had already served in the army for twelve years. A few things can be learned from this example. Gracchus, in Plutarch's narrative, explicitly connects his twelve years in the army with his quaestorship. Gracchus was posted as quaestor to Sardinia, where he served for two years even though he was permitted to return to Rome and be replaced in his office after a year. This makes sense: the quaestors were basically the quartermasters of the army, and young men embarking on their political careers served in this logistical capacity before entering into higher offices. C.'s brother Ti. had similarly served as quaestor on campaign at Numantia under Scipio. At the time this was the typical career path, although the quaestorship was not formally a requirement for higher magistracies until later. Additionally, Plutarch doesn't tell us what C. did during his twelve years of service. Stockton understandably supposes that probably the last two years or so were actually as part of Ti. Gracchus' land commission. Presumably the first couple campaigns were as a cavalryman, followed by a few years as a military tribune.

The military tribunes require separate discussion. The Roman people elected their own military officers, and the events of 88 and 87 demonstrated that they ideologically believed that they had the right to maintain or deprive those commands. Additionally, while Polybius reserves for the senate the ability to dispatch the magistrates on campaign, the existence of several laws (the Lex de provinciis praetoriis, the lex de Termessibus, etc.) passed by the assembly dispatching magistrates and issuing them their military orders shows that at least sometimes the people chose where their commanders were posted (the textual sources attest to this during emergencies, such as 107 and 88). The Roman people also elected their own middle officers, that is the military tribunes. Not all military tribunes were elected--consuls could appoint military tribunes when they needed more, and this seems to have been common enough by the time of Polybius that he actually seems to think that the military tribunes are appointed rather than elected--but the military tribunate was usually the first elected office that a young political hopeful was appointed to. The reasons are obvious: Polybius claims that the military tribunes were required to have either five years of service or ten. If Polybius is correct then the requirement was rarely, if ever, being observed even in his own time. In any case, the military tribunate allowed a young man to gain his required military experience while still establishing himself in the political world by gaining experience in canvassing for office. Even if, by the late Republic if not well before, the "required" ten years of military service were almost never fulfilled, still a candidate who had not done his military service stood no chance, such that there are zero attested examples of quaestors and other senatorial magistrates who had not at least done a couple years as a military tribune.

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