What countries were in the 100 years war and why were they involved?
The basic countries at war were England and France.
France was not a unified country at the beginning of the war. The nobles ran their estates much like separate countries.
Phillip the son of John the Fearless (Duke of Burgundy) allied himself with Henry V and the English in the Anglo-Burgundian Alliance.
In 1422 both Henry V and Charles VI died leaving their thrones to their children. Henry's child was his infant son, and Charles left his throne to Charles VII.
Eventually the French and Phillip (Duke of Burgundy) came to terms and the Anglo-Burgundian Alliance came to an end at the Treaty of Arras. Phillip then began supporting the French which angered the English.
The Hundred Years War ended when the French defeated the English at the Battle of Castillon in 1453.