There is an abundance of historical examples to the contrary. Especially in premodern times, it was very common for commanders and heads of state to fight in the front lines. These rulers and leading figures would start wars partly to pursue their own personal glory, profit, and social advancement, even if it meant risking their lives. Many of them died, were captured, or suffered the loss of their position of power as a direct result of the wars they started.

Perhaps the most telling counter-example to your cited claim is the ancient Greek direct democracies whose citizens voted for themselves to go to war. With the exception of citizens over 60, the section of the population that had voting rights was also liable for military service. That meant every single person who voted for a particular war would also have to fight in it. This fact never stopped Greek states from going to war; in some cases it only made them more prepared to do so.

It often wasn't just a single decision either. The male citizens of Athens, through their support of Perikles, willingly went into the Peloponnesian War in 431 BC; though they tried unsuccessfully to negotiate peace after the first year and the outbreak of a terrible plague, they would eventually vote against making peace with Sparta in 425 BC, and again in 412 and 406 BC during the second stage of the war, when their situation was much more precarious. In other words, even those with extensive personal experience of war were not inclined to put a stop to it unless they feared imminent disaster. The same thing happened in 391 BC during Athens' next war with Sparta.

Incidentally, while Sparta was not a democracy, it did have an assembly of leisure-class citizens, and this body also voted to go to war with Athens. These were the men who had to march out every year to try to provoke the Athenians into a pitched battle; even so, it took until their first defeat (in 425 BC) before they were willing to consider peace.

Some ancient Greek writers even highlighted this as one of the downsides of democracy. The young and inexperienced were always keen for war, they argued, and the poor were eager to make a steady wage by serving abroad. A political system that gave these groups the power to sway an assembly and set policy was always going to be belligerent. They believed it was a feature of oligarchy to be more quiet, reserved, and slow to resort to violence. Whether this is true or just a partisan impression is open to debate, but it is telling that they believed wars were more likely to happen the more potential fighters were involved in the decision-making process.

This is of course leaving aside the problem of leaders starting wars because they wanted to fight in them. The Spartan governor Klearchos, murdered by the Persians in 401 BC, was characterised as philopolemos - a lover of war, never happy unless he had someone to fight. Many other rulers and states in the ancient world behaved in a similar way. We should probably see this as a result of the glorification of war in Greek culture (and other ancient cultures) as well as the brutalisation that followed from long periods of war. If you never expect to live in peace, or even dread the thought of never getting to make your mark as a fighter, there aren't many barriers against the decision to start a war.

I had originally intended this to be a follow-up response to /u/Iphikrates' comment on the (lack of a) secret ballot at Athens, but it's really a separate comment. So here we go:

I'll add to this that we see basically the same thing elsewhere, even where there's a secret ballot. Polybius makes a big deal of the fact that the Roman people, not the senate or the magistrates, make the decision on whether or not to declare war. Polybius even makes this one of the cornerstones of his argument that the citizens' assemblies of the Republic are a stand-in for democracy in his notion of a "mixed" Roman constitution, since the consuls basically don't have a job if the people refuse to sign off on their campaigns. In practice, the people almost always voted for war--almost always.

There's a famous incident in 200 reported by Livy that, when Rome's Greek allies appealed for their aid against the Macedonians, the senate's response to the envoys was deferred until the people's response could be assessed, as only they could formally declare war. Livy tells us the chronology here: the envoys requested aid from the senate, the senate commended the allies for their steadfastness, the consuls drew lots for the provincial commands, and the consul who drew Macedonia (P. Sulpicius) referred the matter of war to the people. Famously, the military assembly rejected the declaration of war (according to Livy, "by almost all the centuries," ab omnibus ferme centuriis), causing Sulpicius to address the people in an ad hoc contio, in which he explained that the Macedonians had made war on the Romans already and that the choice was really to defend at home or to go on the counteroffensive. Sulpicius called a second vote (which had not been done before and was never done again), and this time the military assembly approved the motion to go to war.

The passage is much-discussed, particularly by Henrik Mouritsen, who I think comes to the wrong conclusion that it indicates that only the aristocracy's opinion mattered (the events seem to indicate precisely the opposite). More appropriate is probably Robert Morstein-Marx's suggestion, backed up by a much more systematic look at ballots rejected by the people throughout the course of the Republic, that magistrates didn't propose motions that they weren't reasonably sure the people would approve, for many reasons but in particular because of how embarrassing it was to be defeated at the ballot. If the motion was not obviously popular, magistrates needed to communicate to the people why they ought to support it. That's exactly what Sulpicius does here, in an admittedly rather odd piece of procedure: he finds that the people, worn out from fighting Hannibal, don't seem to understand the point of the war, and communicates it to them (realistically or not). And this makes a certain amount of sense, since the events leading to the declaration of the Second Macedonian War against Philip occurred rather quickly, and there's every reason to expect that the people had not had enough time to digest what was going on.

If we break down the sequence of events further we see some interesting evidence for popular sanction of war. Note how the consuls drew lots for their provinces after the envoys had already asked the Romans to enter war. This, in 200 BC, was normal. During the Republic, consuls were military commanders first and foremost, which is why they were elected by the military assembly drawn up in its centuries, or the military units in which the voters in the assembly were expected to fight. Until C. Gracchus passed a law requiring that the consular assignments be chosen and published prior to the consular elections, the consuls weren't assigned their campaigns until after they'd taken office. The upshot of Gracchus' procedure was that the people now knew for certain where the consuls that they were electing would be posted during the year, which meant that they could more confidently choose the best man for the job. The problem with that was that the older procedure, of selecting provinces shortly before heading out on campaign (since traditionally the consuls left for their provinces in March of their year), allowed the Romans to respond quickly to crises--because of the time between the elections and the consuls' departure for the field, following Gracchus there was a delay between events requiring a military presence and the dispatch of an army. Yet this issue was again resolved with reference to the citizen assemblies. From the late 2nd century on, consuls or praetors could be given special commands through laws passed by the people--usually through the tribal or people's assemblies--that allowed the rapid dispatch of an large army. That's how we get Marius taking over the war against Jugurtha (and being handed the war over Mithridates in 88 that had already been assigned to Sulla), it's how Caesar ends up with Gaul despite the consuls of 59 having originally been given non-military assignments by lot, and it's the purpose behind the inscribed Lex de Provinciis Praetoriis that issues special commands to the praetors commanding provinces in the east.

Conceptually, then, the Roman people both fought its own wars and decided against whom they would be fought and under whose command. But this business about the secret ballot is interesting. In 200 BC when Sulpicius asked the military assembly to redo its vote, voters still voted by acclamation--that is, they were asked by an official how they wanted to vote, and responded verbally. By about the middle of the 2nd century verbal voting had been replaced by the secret ballot, wherein voters were given one tablet containing a yes vote and one containing a no vote and cast their ballots accordingly. Yet, even with the eyes of the aristocracy off of them, voters continued to vote consistently for war--in fact, arguably even more enthusiastically. But the secret ballot opens up another question: who was voting? After all, the majority of Roman soldiers, as late as the emperor Tiberius when conscription was ended in Italy and moved entirely into the provinces, were from rural Italy, particularly the Apennines and the northeast. It's probably not unreasonable to expect that these people weren't often coming to the city to vote on every single war (although the trek of the Marsi under Popaedius to Rome following Livius Drusus' murder is clear enough evidence that large populations could be mobilized for political purposes from rural areas in extremely small windows of time) and that the majority of votes on war were dominated by the urban residents who didn't fight in them. Moreover, by the time of the Social War between one half and two thirds of Roman expeditionary manpower was being drawn from Italian allies who didn't vote (except possibly the Latins through one randomly selected tribe--but this wouldn't have affected the military assembly).

But this is far from evidence that these same wars were not highly popular in the countryside, or that the rural farmers and tribesmen who made up the vast majority of all of the Republic's armies would not have supported them at the ballot. While there's some occasional evidence of highly unpopular wars, these are very unusual, and typically are the result of abnormal circumstances. The most well known example, at least to Roman historians, is the Spanish wars being fought around the 140s and 130s, a decades-long struggle without clear objectives or a front line that rapidly turned into a war of attrition and insurgency that was costly in Roman lives and offered little chance of material gain for the common soldier through loot and plunder. But instances like this are quite rare.

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