Learn Farsi: آنها از حقیقت دفاع نمی‌کردند.

دفاع کردن is a compound verb, so we only conjugate the helper verb کردن (to do/make). We add the negative continuous prefix نمی‌ (nemi-) to the past stem کرد (kard), and finish with the "they" ending ـند (-and) to make نمی‌کردند.

Why is there an از (from) right before حقیقت (truth)?

In English you directly "defend the truth," but the Farsi compound verb دفاع کردن (to defend) always requires the preposition از (from/of) for the object being defended. You can think of it literally as "making defense from something."

Could I attach the نمی‌ prefix to the word دفاع instead?

No. With compound verbs, prefixes like می or نمی‌ only ever attach to the actual verb part (in this case, کردن). The noun or adjective half of the compound verb, like دفاع (defense), always stays separate and unchanged.

What is the exact word order happening in this sentence?

Farsi puts the verb at the very end of the sentence. The subject آنها (they) comes first, the prepositional phrase از حقیقت (for/from the truth) sits in the middle, and the full compound verb دفاع نمی‌کردند (were not defending) closes the sentence.