Learn Spanish (Latin America): Los diplomáticos terminaron por prolongar la disputa, lo que frustró a todos. - The diplomats ended up prolonging the dispute, which frustrated everyone.

The structure terminar por followed by an infinitive is a culminating periphrasis. It emphasizes the final outcome of a long process, translating to ending up doing something. If we just used prolongaron (they prolonged), we would lose the sense that this was the final result of their actions.

Could we use acabar instead of terminar here?

Yes! The phrase acabaron por prolongar means exactly the same thing. In Latin America, terminar por and acabar por are interchangeable when showing how a situation finally concluded.

English uses the -ing form (ended up prolonging). Why does Spanish use the infinitive prolongar here?

While you can say terminaron prolongando using the gerund without a preposition, this specific sentence uses the structure terminar por. In Spanish, prepositions like por must always be followed by the infinitive form of a verb, never the gerund.

Why is lo que used here instead of just que?