Sorry, doesn't count:
Barack Obama has said he considered joining the United States military when he left school but decided not to because the Vietnam war was over and "we weren't engaged in an active military conflict at that point".
The statement is thought to be the first time during the 19-month-long presidential campaign that the Democratic nominee for the White House has indicated he once wanted to serve in uniform. The aspiration was not mentioned in either of his two volumes of memoirs.
"Considering joining" the military is not the same thing as "wanting to serve." If you want to serve, you will. Military service is no predictor of anything when it comes to the presidency. We've had 10 generals -- never mind mere soldiers -- become president, and their record is mixed. Ditto for the non-military-serving presidents. So be honest about it. I considered the Peace Corps, very briefly. I don't deserve any "world citizen" points for the fleeting thought.