Seven Years' War aka The French and Indian War

Here is perhaps where I am most puzzled by the motivations of the French during our revolutionary war.  Britain had backed our protection during the French and Indian War and required more colonial support to continue to protect the 13 colonies.  Yet, less than fifteen years later later, France was on our side, and rather than asserting colonial aspirations of her own, France basically sacrificed blood and treasure on our behalf, presumably to add another adversary to oppose Britain.  Is this then a similar situation as the US supporting Vietnamese "freedom fighters" and a fledgling democracy to oppose the greater influence of China?  It seems that many smaller countries typically become pawns in larger global struggles, but in the case of the US, we had enough talent and geographic distance for long enough to oppose old world influence and our incubation period was a success.

http://people.csail.mit.edu/sfelshin/saintonge/pictures/needham.jpg

http://www.masshist.org/maps/2726_NorthAm/2726_NorthAm.htm#