I am currently a Sophomore in High School and have been wondering for some time now if studying abroad in Japan for my senior year would affect my chances of being accepted into USMA either positively or negatively.
Most of your application credentials are determined by end of Junior year.
Your GPA on application will only include 6th semester [Junior year] and you will have SAT / ACT scores by then as well.
The intangibles are the leadership / athletic components and the reasons for your international interests. i.e.: Were you captain of varsity team junior year only to leave? Is the study abroad a natural extension of ongoing interests or just something you wanted to do? Can you continue participation in athletics / physical training while overseas?
Its not an automatic negative or positive IMHO but be prepared to explain why you chose this in light of your interest in a military leadership career.
[again IMHO 'Something I always wanted to do' might not be the response that gives you the edge.]