Final answer:
France joined the American fight against Britain due to their revolutionary effort against monarchies and based on the Treaty of Alliance. Additionally, France's victory at the Battle of Saratoga convinced them to form a military alliance with the United States.
Step-by-step explanation:
France was willing to join the American fight against Britain primarily because of their revolutionary effort to destroy all monarchies. This was highlighted by the French Revolution, which was seen as a beacon of hope for larger republican movements worldwide.
Additionally, France had a diplomatic alliance with the American colonies based on the Treaty of Alliance. According to the treaty, each country pledged to defend the other in the event of a war with Great Britain.
Furthermore, France's victory at the Battle of Saratoga in 1777 convinced them to recognize American independence and form a military alliance. The French provided the United States with much-needed military support in the form of soldiers, naval forces, and military leaders.