When did the French become involved in the American Revolutionary War?
The US got involved in the Revolutionary War to become a separate country. American history classes will teach that the colonists wearied of 'taxation without representation'. This meant that while they were being taxed (and taxed and taxed and taxed again), they had no voice in the taxation process.
After the French and Indian War was concluded, British soldiers remained in America, and taxes were imposed on Americans to defer the cost of the war. As the taxes were not exorbitant, I think it was the presence of the British soldiers which made many Americans believe they had become more American than British, and they should be a separate country.
The Declaration of Pillnitz was taken by France as a threat to the Revolution and it started the French Revolutionary Wars. It also convinced those involved with the Reign of Terror that Louis XVI was a threat to the Revolution. It had become time for France to answer the Prussia and the Holy Roman Empire with Regicide.