What was the fate of these ideals? I know after the French Revolution, there was a period of reactionary conservatism throughout Europe, up until around 1848, when nationalism, imperialism, and a focus on industrialization took hold, but it seems like the rights of the individual took a temporary step aside, as industrialization and capitalism became the name of the game. What was their role, if any, in WWI, when men readily fought for their respective nations at the beginning, and how did they persist through the disillusionment that came at the tail end of the war?
My history professor told my class that our ability to understand and answer blockbuster question such as this one would be on our final, although not this one in particular; so really, I'm asking for guidance in which direction I should focus my efforts to effectively answer a similar question.
Big question.
If we take 'Enlightenment ideals' to mean 'rights of the individual', we're not going to get very far. Better to think of it this way : Enlightenment thinkers had (for that time) unusually generous ideas about what sorts of people could be said to possess legitimate political identities and inalienable natural rights. Between about 1765 and the collapse of the Revolution, we see high-water marks in anticolonialism, abolitionism, what we can loosely call 'women's rights', and the rights of the poor. If we think about these movements as being supported by a basic belief in the rights of the individual, we're about halfway there. The remainder comes down to the principle of basic equality between individuals and groups.
What happens after the Revolution is that the idea of individual rights basically persists, but legal and philosophical definitions of who counts as an 'individual' possessing those rights gets very much narrower for a while. Notable Enlightenment thinkers like Diderot and Burke generally sympathised with the rights of non-white indigenous peoples, the poor and sometimes women. All of those groups saw significant gains at various points during the Revolution, but then things went kind of pear-shaped and the Terror happened; gains for non-whites, women and the poor were mostly lost after it was all over. In some cases, as with the abolition of slavery and emancipation of French slaves in 1794, it would take many, many decades to recover those gains (1848).
Basically, the Revolution left France with a lot of very traumatic memories and the French ruling class (including, oddly enough, Alexis de Tocqueville) with a deep distrust of 'the people'. Populism would take about 60 years to recover (less in Britain), and while the 'will of the people' was more-or-less out of play, the nation became the primary actor. From the mid-nineteenth century until WWI, there was a school of thought which conferred on European nation-states something like natural rights (Tocqueville advocated this). So, France and Britain built their empires, expressing their national character and saving the brown people of the world from themselves (<- sarcasm).
Between WWI and WWII, the abject hypocrisy of allowing one people to dominate and exploit another in the name of national self-determination really started to make some waves. In part, this is because colonial peoples had fought and died in large numbers on the battlefields of Europe under the flags of nations which denied them the very rights and sovereignty they were fighting for. Partly for this reason anticolonial movements in Europe and nationalist movements in colonial territories pick up steam between the wars. During WWII, colonial territories again sacrifice hugely in an intra-European conflict. After it's all over, we see attempts to expand that concept of full 'individual' to begin including colonial peoples, but this was all too little, too late and the wars of decolonisation finished what European colonial administrations had started.
So, one question might be who (or what) from the late nineteenth century through the mid-twentieth century can be said to have had a fully-expressed political identity and fully-vested individual rights? That group dramatically changed composition over the period you've asked about and it may help you to track the expression of Enlightenment ideals from one end to the other.