I've read that it could have been attributed to the an increase in mass media, and subsequent induction of said masses into the political sphere, or that it could have something to do with the proliferation of easy to manufacture firearms that could arm a populace.
But as far as I know, throughout history, ideology wasn't used as a justification for war by leaders of countries/national entities. And yet from the 20th century, you have the Boer Wars, the Russian Revolution, the Chinese Civil War, the First and Second World Wars, the Korean War, the Indochinese Wars, the Mau Mau uprising, FMLN in El Salvador and FARC in Colombia and all the other Latin American insurgencies, the Bambatha Revolt, heck, the whole Cold War.
Why were these wars for ideology suddenly popping up all over Europe and within European sphere of influence by the turn of the 20th century. And why *not* before?
Thanks for any answers you can give.
I think the premise is wrong, there have been many ideological wars over the past 250 years, and I think to the extent that 250 years is even a decent limit, to a large extent that required a narrow definition of "ideology".
Even excluding wars that had a significant religious justification, which arguably could be considered a type of ideology, and wars related to ethnic self-determination and class, the premise remains wrong.
I think even with the a relatively narrow definition there are many ideological wars prior to the 20th century. Let me make a partial list:
1876 Colombian Civil War, where the two involved factions were the "Liberal Party" and "Conservative Party"
The "Liberal Wars" in 19th century Portugal
The French Revolutionary Wars.
The Revolutions of 1848 all across Europe
The revolt of the Paris Commune.
The US Civil War as well as conflicts that preceded it like Bleeding Kansas.
Greek War of Independence in the 19th century (If ethnic conflict doesn't always qualify as "ideological", surely ethnic conflicts motivated by romantic nationalism qualify?)
Meiji Restoration (While it involved different dynasties, it was really primarily about strong, differing views over how the state should be governed as well as the general national orientation)
American Revolution (Ideology was certainly used as a justification for the Revolutionary War, and it's hard to see an argument that makes the Boer War or WW1 more ideological than the American Revolution)
The English Civil War in the 17th century
Leaving aside these wars which clearly have at least a significant ideological component, there have been innumerable wars justified by religion, or peasant revolts, or conflicts where one or both parties considered their objective to include maintaining their freedom or traditional way of life or form of social organization, all of which can arguably be considered ideological in some fashion.