Webster's Dictionary of Synonyms, first edition (1942) calls accord (in the sense of "agreement") and compact (in the sense of "contract") "analogous" words, but it discusses them in two separate clusters of related terms.
Accord appears with agreement and understanding in a cluster headed by agreement. Here are the relevant comments about accord:
Agreement, accord, understanding are here compared in their political and legal senses. They agree in designating a settlement reached by parties to a dispute or engaged in negotiations. All these terms imply concurrence as to what should be done or not done; all imply reconciliation of differences. ... An accord is, in controversies between governments, an informal agreement; use of this term often implies that all details have not yet been settled or that the terms of the agreement are not yet ready for publication, but that the conditions necessary to a final agreement have been fulfilled. In law, an accord is an agreement between the parties concerned in a case where satisfaction for an injury is demanded. The execution of such an accord bars further litigation.
Compact, meanwhile appears in a much larger cluster with contract, bargain, pact, treaty, entente, convention, cartel, covenant, concordat, indenture, and mise, under the general heading contract:
Contract, bargain, compact, pact, treaty, entente, convention, cartel, covenant, concordat, indenture, mise designate an agreement reached after negotiation and ending in an exchange of promises between the parties concerned. ... A compact is an earnest or solemn exchange of promises, sometimes between state or political groups and often between persons.A compact may be unwritten or undocumented, the only assurance of its execution being the trust which each party places in the word of honor of the other or others. The word is used when a keen sense of the obligation which it imposes is assumed of each of the parties. [Examples omitted.]
Black's Law Dictionary (1968) takes a broader yet more formal view of the meaning of compact than Webster's Dictionary of Synonyms does:
COMPACT, n. An agreement; a contract. [U.S. court case citation omitted.] Usually applied to conventions between nations or sovereign states. [Definitions from hornbooks and case law:] A contract between parties, which creates obligations and rights capable of being enforced, and contemplated as such between the parties, in their distinct and independent characters. [Source citation omitted.] A mutual consent of parties concerned respecting some property or right that is the object of the stipulation, or something that is to be done or forborne. [Source citation omitted.]
On the other hand, Black's acknowledges only the "resolution of an injury" meaning of accord (the meaning that Webster's discusses in the final two sentences of its commentary on accord):
ACCORD, n. A satisfaction agreed upon between the party injuring and the party injured which, when performed, is a bar to all actions upon this account. [Citations omitted.] An agreement to accept, in extinction of an obligation, something different from or less than that to which the person agreeing to accept is entitled. [Citations omitted.]
The most significant difference in meaning between accord and compact as applied to international agreements seems to be that an accord is in some sense preliminary to the final agreement binding the parties; it is an affirmation that, in principle, the governments engaged in negotiation have reached broad agreement on the resolution of the issues before them, but that at least some details remain to be hammered out. A compact, on the other hand, is an exchange of specific agreed-upon commitments more clearly in the nature of a binding contract at international law.