George Washington and his contemporaries were given a classical liberal education. Many at least read Greek or Latin. I have always looked at the Founding Fathers as looking backwards to Roman and Greek history and literature. In this case I did a bit of digging and found a synopsis to a play called the 'the Archanians' in which a separate peace with Sparta is arranged through a clique within the city of Athens during the Peloponnesian War.

Washington also may have read the speeches of Cicero and other Romans and certainly knew his Roman history and could also have been easily referring to the times of Julius Caesar and the end of the Republic in which for example Brutus received foreign support in his attempt to raise another army to battle Augustus.

The president could also have been contemplating the period of the Oliver Cromwell and the radical changes which were put in place under the commonwealth by the Puritans.

He was also probably thinking about the recent issues involving the supporters and non supporters of the French Revolution which was creating visible symbols of divisiveness between the infant political parties in the country. The XYZ affair was only a few years away, and the early diplomacy of the nation was riddled with partisan support for one country over another.

I doubt a clearly defined document outlining his exact thoughts on the matter could be produced, but have wondered myself at the Presidents exact thoughts when he mentioned this in his final speech.

A follow-up question: is it possible that he had in mind the events which had occurred over the preceding decades in Poland-Lithuania, whereby Russia manipulated the Commonwealth's internal politics in order to paralyze and ultimately dismantle the country?