People here downvote all the time once discussions are political. Don't bother.

What I wrote applies more to civil war than to war between nations (where citizens can be forced to war by governments). Of course, in reality the line between war between nations and internal civil war is often blurred - civil wars and wars are often mixed up and neighboring nations often support one side with money, weapons, troops etc. Furthermore realities of war can force people to take side once the conflict is ongoing, if nothing else, then to survive.

Now, with that in mind, prior to WWI you had a large generation of poor young German and Austrian men, and a large generation of poor young French, Russian, and British men. Today you don't.

Also, visit a European house or apartment from 1850-1900, which most people still lived in around WWI. One that is used as a museum so that it hasn't been renovated. You'll see that they were significantly less comfortable than today. No AC, no radiators, often no toilets (but a loo outside), usually no showers. Heat was from coal briquettes in a stove near the center of the house which meant constant draft and bad air. Cooking happened from gas, briquette or wooden fueled stove. There was no fridge or freezer but usually a small room that was cooler and where food would be stored. The house had little or no insulation, the windows had one layer of glass. There would often be electricity, but it would only be for the light bulbs. Radios or phones were still not common, hot water was still not a thing everybody had. There might be hens in the yard and milk was delivered in bottles. Most of this, except the radios and telephones, was true for 1939 as well.

Having been in the army myself, it always strikes me how much closer life before modern conveniences was to army life. Take 25 year old Hermann, a single guy from 1914, and put him in a military camp. He would not experience such a huge drop in comfort as you or I would before the fighting begun. He would eat, drink, sleep, keep warm etc. much in the same way as if he was home. He would entertain himself with books, songs etc. much in the same way. He would be surrounded by men just as he likely would at home or work or university. This was in the beginning of woman's emancipation so he would probably not miss out on a dating scene, going out with girls, male/female social life etc. For many, life meant long hours of hard manual labor in factories or farms, not less hard than the manual work in military. No vacation, no weekends (but a Sunday). His life outside of military was not that much better. Until the fighting begun, of course.

Today, joining a primitive militia/guerrilla group would mean a huge decline in quality of life of an 18 year old from Barcelona. Take 25 year old Jordi from Barcelona. He graduated as a graphical designer some months ago and spends 2 hours a day playing Counter Strike and 3 hours on his iPhone7 swiping left and right. Sometimes he looks for jobs online. He has female friends, and a dating life. He is used to walking to the metro, but otherwise doesn't get much exercise. He still lives with his parents and will likely do so for a few years more unless he gets himself together and turn that computer interest into something useful. Life has its challenges, but it sure isn't that tough. He has never killed anything he ate. He has never done much manual labor. Right now he's concerned if he will have enough money to join his friends skiing in the Pyrenees in a few months and what he's going to tell the date tonight about his job situation. He sure isn't worried about food on the table.

Might Jordi join the protest on Placa Catalunya? Yes.

Would he take up arms and hide in a forest in the low mountains west of Zaragoza waiting to ambush the approaching castellano troops? I wouldn't say never, but I just can't see it happening. Jordi loves the idea of Catalonia but he also loves his life and camping is not really for him. Plus he has a date tonight.

There is simply a greater difference between Western "working class" or middle class life and the outdoor, offline, off dating manual labor life of a soldier, than there was before.