Tribalism. Our tribe is good, their tribe is bad. If you are not with your tribe, you are a traitor, so people are pressurized into this, and start believing lies about our glorious pride and wise leaders.
That's often the case when it's a battle over shit like oil but for instance when Germany attacked Poland, it's the Poles' primary responsibility to fight for what is theirs. "Let the people who declared war be on the front line" okay well in this example that's Germany and you're going to lose everything your family ever worked for.
I'm going to go with based. I would not have wanted to be a conscript with the rank of private in any army at this time period. You'll be badly treated disposable cannon fodder and you'll be stuck in that situation until the war ends, until you get taken prisoner, until you desert, or until you died.
Indeed, is there more you want to comment? Or is there more you would like to know about the 1930s in Europe? Now I understand that I am taking a liberty with the assumption that OP is referring to European heritage, but I could direct you to a book or two to expand your knowledge of history
My grandfather did the exact same thing. But he was in conquered France and did it to avoid being drafted by the Nazis.
Edit: There were also a stories of other people in the village finding other ways to avoid the Nazi draft. One of them pretended to be deaf and managed to avoid falling for a few tricks the Germans played on him in an attempt to expose him, such as dropping a coin behind his back. Another one reported to the draft medical examination after purposefully biking over an insanely long distance. When he arrived he was completely exhausted and the doctor deemed him too anemic for military service. All of those in the village who couldn't dodge the draft were sent as cannon fodder on the eastern front and never returned, except for one "lucky" individual who lost a leg.
My favorite is the Battle of El Sauce, at which the International Consortium of Spanish-speaking Countries defeated the Yum! Brands Taco Bell Corporate Mercenaries to save the word "salsa" from extinction
Doesn't it depend on the war? A lot of good allied soldiers died defeating the Nazis, but a lot of good would-be Nazis also found ways to avoid service.
Every draft dodger in the invading nation is one less soldier you need to defend against.