The France of today has evolved through 2500 years of history. During that time Europe was occupied by various invaders, and the alliances of different empires changed borders. In later years territories were acquired by royal marriage, and even bought and sold.
Some areas changed nationality several times, particularly those adjacent to the ever changing frontiers.
France itself, was an invader at various times and the borders of France expanded to accomodate conquered territories.
Savoie finally became part of France in 1860, Alsace was returned to France in 1919. Hope this helps.
I believe it was near the end of the 400's.