Look, I read a lot of history, but there are gaps in my knowledge. So let me ask you all this.
In all the history of democracy, whether the leaders were actually elected or "elected" in fraudulent, rigged election processes? Once the elected (or "elected") leader declares a war, whether or not the laws of his or her country gives war-making authority to that office? Once the first soldiers, in obedience to that order whether they should have obeyed or not, crosses the enemy (or "enemy") border? Has any war ever been stopped, short of being won or lost? Ever?
In 1860, there were an awful lot of Yankees who recognized all of Dixie as, well, frankly, what it still is today: a dead weight hanging around this country's neck, acre after acre of completely useless land full of mostly useless people, a net recipient of tax revenue rather than a net contributor to the country since shortly after Independence. People who said that if Dixie wanted to secede from the Union, then never mind the Constitutional niceties, let them go, and good riddance to bad garbage, certainly not worth spending one life or one dollar (let alone millions of lives and the entire federal treasury) to keep. Pro-war Unionists, once the war started, called these people Copperheads, and not a few Americans lost everything they owned to court-ordered confiscation and either went to prison or fled into exile after being convicted of sedition, for speaking out against the war once it had started. Many of them paid very nearly the ultimate sacrifice to try to stop their country from waging a murderous and unjust war. I also know that there were anti-war riots in several northern cities, most famously the anti-Draft riots in New York City. Did any of the journalists, anti-war politicians, popular authors, or rioters succeed in stopping the Civil War?
From 1913 to 1917, The War in Europe was the hot-button political issue in the United States; Woodrow Wilson only won the presidency in 1916 by promising to keep us out of it. It was the clearly and unambiguously polled opinion of the vast majority of American voters that it wasn't worth one American life or one American dollar to get involved, and to this day, I still don't claim to understand what American interest the Wilson administration thought they were pursuing when they used the passengers of the Lusitania as human shields for an illegal arms shipment to England. I do know that there were labor activists in the US who saw through this, who got the reporters for many newspapers to accurately report that the German High Command had warned the passengers of the Lusitania that they were being used as human shields before the ship sailed. Did the anti-war voters, or the labor activists, or the journalists manage to stop US involvement in World War I? (Too bad they didn't, too; had they succeeded, nobody but a handful of specialist historians would ever have heard of Adolph Hitler. Or, for that matter, Osama bin Laden.)
From when Supreme Fuhrer Hitler renounced the end of WWI peace treaty and began re-arming in 1936 and threatening aggression against his neighbors, there were many thousands of pacifists and other anti-war activists who opposed him. Once the war broke out, some of them, having forseen the disaster to come, went so far as to sabotage their own country's war efforts and even attempted to assassinate the Fuhrer. Did they manage to stop WWII?
I know that a ton of old hippies claim that their anti-war protests brought down the Vietnam War, but actual professional historians who've analyzed the war conclude that on the contrary, the anti-war demonstrators may have actually prolonged the war, by making the anti-war side look so unattractive and anti-American that they made it infeasible for both Johnson and Nixon to accept defeat. Thousands of dead later, and more than four years after the peak of the demonstrations and riots, the war ended not when the anti-war forces tried to stop it, but when the US Army was defeated.
When the Soviet Union invaded Afghanistan, there were few people who questioned the war, but never was anyone more powerless and helpless than an anti-war Russian in the USSR. As the death toll began to mount, though, even in the old Soviet Union people risked their very lives to protest, to demand that the Communist Party explain what was so important about Afghanistan that so many of their friends and family members had to die there. And they kept right on protesting, quietly and loudly, and did they stop the war that way? As if. Nor did it even stop the war when the Soviet Army's returning soldiers began questioning the war themselves, in public. No, trust me, long before that war ended, there were tens of thousands of Russians, maybe even hundreds of thousands of them, trying to end the war. Did they? Fat chance; that war ground on until the CIA-backed mujahadeen forced the Soviet Army to retreat.
Show me a counter example, if you know one. Show me even one time when, clearly and unambiguously, political opponents of one of their own country's aggressive wars have ever managed to stop that war short of defeat, victory, or total collapse of their own government's claim to legitimacy for other reasons. You want to stop the Iraq War, soon? So do I. But now that I think of it, the more I realize that if I'd paid enough attention to history, maybe I would have seen that there are only two ways to end the Iraq War, soon: in victory, soon, or in defeat, soon. Or okay, maybe an even more implausible third way, in a collapse of the United States into total violent anarchy so thorough that the government can't even patrol its own streets, let alone prolong a war. If you know another way that has ever succeeded before, I'm all ears.