[Elrac]

The "de la" means "some" or "some of," while "la" alone just means "the".

"Folie" (folly) is an amorphous, uncounted quantity, like water or sand. War is a one-of-a-kind entity, like "Peltier effect." So you speak of "some water" but "the Peltier effect."

When thirsty, you could say that "I want to drink the water," but unless there was an antecedent already set up in the conversation, it would sound a bit silly.

[gruseom]

Interesting. I know the definitional difference between de and la and I know that de applies to indefinite things and so on. But the rules don't help here because to me (an English speaker with rudimentary French) la folie and la guerre aren't in different categories of definiteness to begin with. In English, war in general is just war, no different from madness in general being just madness. Thus I would never have constructed the sentence that way, and even after you explain it it still seems strange.

Once you tell me that la guerre is definite and la folie amorphous, then sure, I'd know to use the articles that way. But I would never have guessed this. It seems you have to know how the nouns feel to a French speaker; you can't simply compute the right answer from the concepts.