Namely, the invasion of Poland by the Nazis, which lead to World War II. Usually, the invasion of Poland is presented as something that was purely the product of Nazi plans for expansion, and so it was on the highest levels. But the invasion of Poland that was sold to the people of the Third Reich was something different. Poland after World War I and the gaining of statehood had a considerable German population, the consequence of the Polish state being divided up between Germany, Austria, and Russia for a considerable amount of time. The propaganda that was put out was that Germans in Poland were being harassed and threatened, that the Poles themselves were persecuting them and that they needed help to avoid disaster. Hence, an invasion by Germany was sold not only as a revanchist concept of regaining territory but also as a humanitarian mission of sorts in order to protect the German minority from the "Slavic hordes". Ultimately, the invasion of Poland was triggered by a manufactured incident on the border. Seeing that direct provocation of Polish border police wasn't working German commandos dressed in Polish uniforms descended on a German border outpost and slaughtered the people inside, therefore creating the pretext for going to war.
Although we all know how fucked up and terrible the Third Reich was, becoming synonymous with pure evil, the Nazis appealed at times to themes that have popped up again and again in the history of belligerent nations who want to provoke war for their own wants.
So when people express the idea that this invasion or that invasion has to happen to preserve a noble ideal in the country in question, warning bells should go off immediately, because war hardly ever happens for a noble ideal. The second world war only happened because of the vast expansion of Nazi Germany and Imperial Japan. If people had really wanted to stop Nazism solely for the sake of stopping Nazism they wouldn't have waited until it got so far along in its plan. But, as Clausewitz says, war is politics extended by other means, and nobility of purpose is a convenient blind for covering that up.