Its not so much an anti-war thing as it is a political power thing. Wars were immensely lucrative and it was speculated that Caesar, who was broke, would try to start one. For political reasons, Caesar's enemies did not want to see him with the wealth of Gaul and a few battle-hardened legions at his back (well-founded concerns, as it turned out). With his massive debts and no war, his political career would be over. To ensure exactly that, the Senate forbade him to 1) raise additional legions, while assigning too few troops to fight a war in Gaul and 2) start or fight any wars in Gaul. Caesar casually disregarded both those directives; if he didn't, he was ruined and if he did, there was a chance the Senate would try him for crimes against the state. Again, a good lesson about what to expect if you backed the man, a consummate gambler, into a corner. But I digress. Caesar raised the troops he needed, provoked a war in Gaul (NOT a Roman province at the time), and relied on his thrilling dispatches back to Rome (and a flood of plunder) to keep the public happy and his enemies at bay. When his term ended and he was eligible for prosecution, powerful forces in the Senate (the moral authority of Cicero and Cato, with their unimpeachable Republican credentials, backed by the hard power of Pompey, with his extraordinary wealth and deep reserves of loyal veterans at his call if needed; odd bedfellows to be sure, but Cato and Cicero considered Pompey the more manageable, if not lesser, of the two evils)called him to account for his illegal actions. Backed into a corner with his loyal 13th Gemina Legion, he did exactly what he'd done since he was a boy fleeing Sulla's hitmen; staked everything on a single move and came out swinging. (Rubicon, Tom Holland)* edited to address Cato's role, in the context of your question*

Yes the concept would have existed, but it would have meant something different. We must of course make a distinction between unjust war, or war begun without good reason, and illegal war, which is war begun or conducted in violation of some law, whether this is international law or domestic law. Both concepts would've existed among the Romans of the 1st Century, B.C. The injustice of Caesar's war would have been, and was, the subject of great debate. Much of Caesar's writing in De Bello Gallico is in defense of his actions and legitimizing his aggression. Did Caesar really have a good reason for attacking the various tribes of the Gauls? Cato and many of Caesar's enemies said no, and that even in the case of the Helvetii Caesar should have been content with simply repulsing them, not with marching out of his province (of course the province of Transalpine Gaul didn't really have very well-defined boundaries, so Caesar could simply claim, and did, that he was maintaining order within his province). What might have made it illegal? Modern wars are usually deemed illegal when they violate agreed-upon rules, such as those the UN stipulates. They can also be deemed illegal if they are undeclared or are declared without proper legal reason. Roman wars were conducted by magistrates holding imperium, and therefore with an army. The senate had no right to declare war and theoretically magistrates did not have to consult with the senate before engaging in warfare (since the senate had no authority to pass legislation or command magistrates). Within the limits of the imperium that went along with his office a magistrate had the right to do whatever he wanted with that army. So what made his military actions illegal? If he violated a law, obviously. The most obvious example would be Caesar's civil war, which was begun due to his illegal action of entering Italy with an army, where his imperium as a proconsular governor did not hold (although Caesar argued that his enemies had also violated the law by essentially declaring him an outlaw for having charged him with violating an illegal law. Yeah, confusing shit). But if Caesar had, for example, marched into Spain during his governorship to fight the Lusitanii that would have been an illegal war, since he had no right to lead an army outside of his province. So was Caesar's war in Gaul illegal? Debatable, but there's not really anything that directly violates any laws. What is possibly illegal was that Caesar's magistracy in Gaul was an extraordinary one, lasting for eight years, when a normal proconsulship only lasted one. Roman magistrates sometimes had their magistracies extended if their commands were in crisis, but by 56, when the Council of Luca extended Caesar's proconsulship for another five years, Transalpine Gaul was by any definition safe--Caesar just wanted to finish subjugating it so he could invade Britain (although of course he ended up staying in Gaul longer than he expected because of Vercingetorix later on). The only reason the decision reached at Luca flew was because Pompey was behind it and Crassus agreed not to interfere, so together Caesar and Pompey were able to overwhelm their opponents the way they overwhelmed them to get the Eastern Settlement and Caesar's proconsulship (the senate wanted to grant Caesar an extraordinary magistracy over the trees of Italy instead of granting him the power of a proconsulship) passed. So in the eyes of people like Cicero it was an illegal bit of legislation that allowed Caesar to stay in Gaul, and a debatably illegal piece of legislation that allowed him to be there in the first place, which only passed because of an illegal political agreement which has always reminded me of a mafia deal.

Were there anti-imperialist sentiments, or sentiments against conquerors in Rome? Absolutely, although they're rather different from what we're used to. Imperialism in our sense of the word is, well, a bit anachronistic to say the least. During the Principate there was certainly sentiment against people going out to try to conquer territory, at least among the higher-ups, who regarded the actions of private magistrates without the counsel of the emperor as suspect. During the 1st Century, B.C., though, things are different, and a bit odd. It's a defining feature of the political and social climate (among the senatorial class those two are the same thing) of Rome that it was dominated by a very small group of politically influential people who were incredibly competitive with one another. With the sudden expansion of their empire following the successful conclusion of the Punic and Macedonian Wars and the unprecedented wealth and opportunities for both wealth and power that were pouring into the laps of this small group of men (which was rapidly growing as more and more people outside the club tried desperately to punch their way in) the governing bodies of Rome found themselves in a crisis. The senate was forced to rely on individuals to deal with the constant threats to their provinces and so forth, and were forced to not only put enormous power in the hands of these individuals, but also gave them the opportunity to amass unimaginably great wealth, power, and influence. Yet at the same time the senatorial class was intensely distrustful of anyone (even someone like Cicero who supported them throughout his career, which was based on the defense of the constitution) who gained large amounts of personal power and influence. So since conquerors could not only gain huge amounts of influence, wealth, and power the senatorial order immediately would begin attacking them. However, I hesitate to call this an anti-war sentiment, because it was not something restricted to conquerors. Individuals like Cicero or Clodius who gained enough power to be seen as threatening the state were also attacked, even though they had no military conquest under their belts. But as military prowess was the easiest way to gain this power it's somewhat appropriate

They absolutely did have an idea of a "legal" war. Battles in the ancient world were more often than not pitched battles, army vs. army in a given area. There were ways of conducting warfare in Roman times, and they were unbelievably good at it. War and peace was also declared, if my memory serves. That makes it about as legal as it can be.

A "just" war is a much more loaded term. That kind of thing comes down to perspective, but in the case of Caesar, he essentially led an army to Gaul to avoid being persecuted for his massive debts that he was dodging (since you can't prosecute someone on campaign). So was it "just?" If you find empire building a just cause, then absolutely.

Granted, I've never watched the show, but that may be what it was referring to. To expand was to be part of the Roman Empire, especially from Caesar's time until around Hadrian's.

I've never heard any anti-war sentiments spoken against Caesar's Germanic campaigns. Whether that was from scholars not wishing to offend the man, or Romans being so used to warfare that they embraced far-away conflicts with greater equanimity than we do today. My personal opinion was for the latter. I cannot remember one single piece of anti-war or empire-building works in Roman times, by a Roman. They valued themselves conquerors and warriors.

Here's a link to the scene OP is referring to: http://youtu.be/oGvGGyLn9Os

To add to the responses here, it is perhaps better to think of Caesar as exceeding his portfolio. The problem, after all, was his warring against Germany.

I'd like to ask a follow-up question after reading the other responses in this thread. I know the Romans had an archaic method of declaring war which involved symbolically hurling a spear into "enemy" territory. Since I do not believe that this ceremony was performed for Caesar's Gallic Wars, couldn't the war have been deemed undeclared, and therefore illegal?

cato didnt really like Ceaser. He may have been divided on him, at least publicly or privately, but it's not true which one is which really.

-Cato the Younger, Plutarch, Lives