No two words have the same meaning so would have been treated as synonyms. But for their uniqueness there is a difference.

An agreement is a signed document document. An agreement is considered a final agreement. The boundary of a certain territory can be defined by an agreement.

Vocabulary. Co says: “a legal document summarizing the agreement between parties”

A compact is an agreement that normally includes promises by party to perform in certain ways under certain circumstances. It may be a decision that nation X will protect nation Y from attack.

Vocabulary. com says: “to execute an agreement between two or more parties by written agreement between their

governments” Agreements are reached, compacts are

formed.