In a democracy, the people elect politicians, and politicians enact policies that have real world consequences. Let us explore the moral implications by considering the following scenarios.
True Democracy Declares Unjust War on Innocent Country
Let's say that the politicians of Salaria decide to go to war, and in this war, millions of innocent people are killed, maimed and displaced. For this scenario, let us assume that the war is completely unjustified. Who in the nation of Salaria is morally responsible? (Note we're not talking about international law. We're only talking about the ethics).
If Salaria is, indeed, a true democracy, then the answer is obvious. The agents of the state who participated in the war, the politicians, and everyone who voted for the war-mongering politicians are morally responsible. If the voters never voted, the politicians would have never declared war. If the politicians never declared war, the agents of the state would have never killed those innocent people. A led to B, which led to C. There is a direct causal link with intent. C is a direct result of both A and B.
False Democracy Declares Unjust War on Innocent Country
If politicians consistently promise one thing and then do another, then the Salarians essentially have no influence over their policians. In this scenario, who the voters elect is determined by a rigged process; and even if the politician really is what the people voted for, the politician will end up listening to special interests instead of the people. In this scenario, there is no direct causal link between the voters and the politician's decision to declare war. A does not lead to B, therefore A does not lead to C. Therefore, the only people who are morally responsible for the war are the politicians, their puppet masters, and the agents of the state.
If Salaria were a dictatorship, then the above analysis would also apply.
Constitutional Republic Declares Unjust War on Innocent Country
In a constitutional republic, elected politicians can only act within the strict confines defined by the rules of the constitution. In this scenario, voters cannot be held morally responsible for a politician who acts outside the bounds permitted by the constitution. On the other hand, if the politician acts within the constitution and follows all the necessary rules for declaring war, then the voters who voted for that politician are morally responsible.
Now let's make things a little more muddy with this question:
If a rogue nation causes the death of millions of innocents, how do you hunt down and punish only the people who are morally responsible without causing any collateral damage? How would you even prove moral responsibility?
Answer: it's impossible. This is why we throw around the term 'collateral damage.' Collateral damage is a collectivist term. It acknowledges the fact that we cannot discern one individual from another, so we accept that some innocent people may die just for the fact that they belong to the collective 'other'.
The idea of a nation state forces all of us to be judged and punished as members of a collective, and it forces us to judge and punish others as members of a collective. When millions and millions of people work in concert to fuel a war machine that's killing another group of people, you cannot untangle the web of moral responsibility, and you cannot hurt the war machine without hurting the innocents that form the individual cogs of that war machine.
If you believe that we are individuals and not members of a collective, you need to question the very idea of a nation state; and if you insist that there should be a nation state, you need to question whether or not it should be allowed to invade another country. If you insist that nation states should exist and that they should be able to invade when necessary, how do you find the right balance between morality and practicality?