I agree.... I think the big difference in the modern era, is that you really have to deal with public opinion in some way, even in states that aren't necessarily democracies - though in non-democratic places you don't have to get any sort of vote, except for the party or some inner circle, depending on the type of system. Germans before they began to invade their "territory" had a fair amount of propaganda aimed at "Aryan" germans in order to justify their actions. Likewise, look at the difference between how the Tzarist state went to war vs. the soviet state. Part of the reason that the Tzarist state pulled out was over popular unrest in part mobilized by Russian involvement in the war. Once the Soviets were pulled into WW2, they had a pretty heavy propaganda campaign, though I think the war was popular enough that they didn't need it that much, because Germany was dumb enough to invade in (what was it, like August or was it July? But what were they thinking). Plus, we had a fair amount of pro-Soviet propaganda during the war as well, interestingly enough.
You can also compare what happened in the Civil War (where you have not too few draft riots in the north from democrats who didn't support the war) and the first world war, where you had a concerted effort to "inform" and persuade the population into a war that Wilson had campaigned against. They created a committee, the Committee on Public Information to create support for the war:
I doubt that when, say the Ottoman Empire in the 16th or 17th century really felt the need to have a propaganda campaign amongst the peasantry in the Balkans or anatolia to support a war effort... or say the Russian Empire or even the French and English prior to the French revolution. Though I guess you could argue that the various religious wars (100 years war and the 30 years war could be classified as using "propaganda" since it was aimed at people of the "wrong" religion but they didn't need consent is my point) They didn't need consent, they could do largely as they pleased in that regards.
It's interesting how things have changed in that regard.