If I loose my US citizenship by joining a foreign military such as the French Legion will i be able to get it back ever again

If you lose your U.S. citizenship for joining foreign military armed forces, you will need to start again by applying for U.S. resident status if you want to return to a residence the U.S.

Perhaps, not! If you are unable to get into the US Armed forces, then the same sorts of bars to service may exist for any international armed service. You can have fewer rights if you 'can' train for a foreign military. It may be best to do a self-evaluation before you approach another recruiter. For this reason, you may want to review why you can't be recruited. This may not be an immigration question. The only issue is whether you must formally renounce your US Citizenship. This is unclear to an attorney who practices U. S. Immigration Law, since it concerns current French Military Law. If you must do so, then there may be other complications and consequences. These may be based, in part, upon taxation. There may be other reasons for U. S. Military service disqualification, such as age, background check or other impairments to name a few. A recruiter must be polite to those who mean well, but may have challenges that make military service inappropriate at the time they approach the recruiter. Once you formally renounce your U. S. Citizenship before US Diplomatic Officials, then it can be extremely difficult to get it back. There may be better ways to serve Our Country, whether it is by improving your skills through training in a career that is in demand. Also, you may want to commit yourself the next employer and to a job that you enjoy, whether it be for a company or government employer. Good luck.

And, this will cause a problem should you want to join a profession or military that requires a security clearance upon return to the US.

As a general rule, if you voluntarily renounce your US citizenship as one must do , for example tojoin the FFL, your chances of getting it back are slim to none. Likely best to check with the Department of State about the details.