Good question. For most of those years (goes for both wars) they just straight up lied about how bad things were going. Then there's the propaganda. Holy hell, so much propaganda. Like not saying you're wrong by asking that question but there are ways of selling unpopular wars.

Added to this, capitalists fucking LOVE wars. And without breaking capitalism all they're really going to do about the majority coming out against war is say 'yeah, what the fuck are you gonna do about it then?'

How do you explain the US wars in Vietnam and Afghanistan?

The Americans were asking for it. Did you see how they were dressed?

Am I? In all those long conflicts the population was at first at least indifferent(Like the US in the Vietnam war or USSR in Afghanistan war) or outright supportive(Like the US in their Afghanistan war) and after the public opinion changed the wars quickly ended. On the other hand the tsar was deposed after less than 3 years of WWI.