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Crusades: A series of invasions. A prolonged war.
No, the word prolonged is not an adverb.The word prolonged is a verb.The nearest adverb for the word "prolonged" is prolongedly.
prolonged
The prolonged pain was unbearable.He prolonged the date as long as he could because he didn't want to have to say goodbye.The so-called shortcut actually prolonged the journey by an hour.
Examples:
→ Her life could be prolonged, but what would its quality be?→ They prolonged the meeting in order to discuss the point.