Why would the French and other foreign countries decide to join the Americans in their fight against the British?

They believed that by joining america they were fighting a common enemy

Explanation:

The main allies here were france, spain, and the Netherlands.

France and spain had already become enemies with britain, and America has shown that they had the ability to win wars.

Britain had already assumed central power in Europe and the rest of the world. France and Spain saw them as enemies. By aiding America, they were also trying to bring down their enemy.

France and spain were also being revengeful as they lost the sevens years war against britain. By aiding America they also wanted to regain lost territories.

Some of the foreigners were aiding America to be free from british rule.