Read the sentence. Washington knew that he had to do something to protect the fort from the French or it would fall into their hands. Which pronoun in the sentence has no specific antecedent?

The pronoun that has no specific antecedent in the sentence is Something because is an indefinite pronoun and the indefinite pronouns do not require specific antecedents which means that those ones are used in place of nouns for people, things or amounts that are unknown or unnamed. They are: all, another, any, anybody, anyone, anything, both, each, either, enough, everybody, everyone, everything, few, fewer, less, little, many, more, most, much, neither, nobody, no one, nothing, none, one, other, others, several, some, somebody, someone, something, such, and they (people in general). So for the case of the sentence in mention, the Pronoun is Something.