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Most Branches of the service will accept someone who wants to enlsit, but you must first pass a Physical examination, Psychological Examination, meet Educational requirements and other requirements deemed necessary to be come part of the military.
All Countries have a set of requirements that have to be met to even be considered for a position in the Armed Forces. Most follow the requirements that are listed in the link below.
This link goes over some that the US Government have set up to join the Armed Forces: http://usmilitary.about.com/od/joiningthemilitary/a/enlstandards.htm
Since the chance of terrorism, the Governments have put stringent constraints on who can become a part of the Armed Services.