It is complicated.

Philip IV was King of France from 1285 - 1314. He had three sons and one daughter.

When he died, his eldest son became King Louis X. Louis died just 2 years later. His only son, John I was born after Louis died, but died himself 5 days after being born.

Who now should rule France? Louis X's daughter Joan, or Louis' next brother Philip? Philip became King Philip V, and the reason given was 'Salic Law' (a law which was either rediscovered from the 5th century, or made up to justify the decision). Under 'Salic Law', it was proclaimed that a woman could not inherit the throne.

Philip V died in 1322 without children. His younger brother then became King Charles IV. Charles IV died in 1328, also without living children.

That is when King Edward III of England claimed the throne of France. His mother was Isabella of France, sister of Kings Louis X, Philip V, and Charles IV.

Edward III maintained that even if 'Salic Law' prohibited a woman from becoming monarch of France, it didn't prohibit her son from becoming King of France. He was the closest heir to the throne, and therefore the rightful King.

The French (not wanting to be ruled by the King of England) disagreed. They interpreted 'Salic Law' as forbidding not just the inheritance of the throne by a woman, but as forbidding the passing down of the inheritance through a female line. They therefore crowned the cousin of the three last kings (and of Edward of England's mother) as King Philip VI of France in 1328.

King Edward III did not accept the validity of this decision. He styled himself as King of England and France, and the course of future conflict was established. In 1337, Edward III refused to pay homage to Philip VI for his lands in Aquitaine and the Hundred Years' War was begun.

Then it got even more complex.

Leaving out a few intervening details, in the Treaty of Troyes in 1420, the English recognized Charles VI as King of France and the French recognized King Henry V of England as his heir (cutting Charles VI's son, the Dauphin, out of the succession). This treaty was rather forced on the French by Henry Vs military victories.

In 1422, both Charles VI and Henry V died. At this point, Henry VI, the infant son of Henry V and Charles VI's daughter (the marriage had been another outcome of the Peace at Troyes) became King of England and France - The only English King to actually rule France (not personally, of course, he was an infant).

The Dauphin was upset by this course of events. With the help of Joan of Arc, he rebelled against the Treaty of Troyes and had himself crowned King Charles VII of France in 1429. By 1436 he had re-taken Paris. By 1453, he had driven the English out of all of France except Calais and the Channel Islands (which Islands are today the only French possessions remaining to the English Crown).

(If King Henry V had not died of dysentery at the young age of 35, things might have turned out differently.)

Needless to say, the Kings (and Queens - England did not adhere to 'Salic Law') of England continued to claim the crown of France for centuries thereafter.

http://web.cn.edu/kwheeler/Hundred_Years.html

http://en.wikipedia.org/wiki/English_claims_to_the_French_throne