What was the primary cause of the hundred years war?

The claim to the french throne for ruling France.

Explanation:

The series of conflict between France and England over the succession of the French throne is known as Hundred years war. The war lasted form 1337 to 1453( For 116 years). Although small disputes have been going on between France and England for years the dispute for the french throne started when Charles sixth of France died without an heir, the french king claimed the french throne because as per the Salix law women were not allowed to rule. King Edward's third of England also claimed the French throne. So we can say that the claim to rule France led to a hundred years war.