What prompted france to sign a formal alliance with the colonies during the american revolution?

Battle of Saratoga

Explanation:

The battle of Saratoga was a crucial event for the open Reaction from France,  in which the Americans. In that battle American forces under the command of General Horatio Gates, claimed the first great victory over the English army commanded by Lord Burgoyne.

Victory at Saratoga is a vital milestone in the American War of Independence, as France, encouraged by that victory, decided on February 6, 1778 to create alliance with the United States and declared war on Britain accordingly.

The agreement was signed between the three commissioners of the United States and the French government. With this agreement, France became the first European country to recognize a new emerging young nation. Benjamin Franklin became the first US ambassador to France, and did so until his return to America in 1785. After 1778, Britain and France conducted war operations both on land and at sea.

answer; (a) he was a u.s senator for 38 years;