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 used maybe 5 Years And Then Join the French military academy and Become a French army officer In The. I dont know Which to something, I Will Have a BA in History and Economics by this year. I love the French culture, Mentality, and I do not know government.So What to do any ideas.