how does one become a officer in the french army after serving in the legion? Can I attend Saint Cyr?

I have a choice to become a officer in the United States army, or go join the french foreign legion serve 5 years and then maybe join the french military academy and become a officer in the french army. I dont know which to chose, I will have a BA in history and economics by this year. I love the French culture, Mentality, and government. So I do not know what to do any ideas.