、。
She couldn't understand why they had fought.
Literal
She [topic-は] why, they [subject-が] fought [explanatory-のか] couldn't-understand.
An embedded wh-question (なぜ彼らが戦ったのか, 'why they had fought') functions as the implied object of 理解できなかった ('couldn't understand'). The のか version of the embedded question carries a slightly heavier, more pondering tone than the bare か form. The comma after なぜ is unusual — it visually highlights the interrogative as the topic of her bewilderment, almost as if the なぜ stands alone in her thought. Note the が→は preserved here in the embedded clause: 彼らが戦った, with that が-marked subject contained inside the larger question phrase.