Your character was approached by a recruiter. Perhaps it was minutes ago, hours, weeks, months, even years ago. The recruiter informed your character that they possessed a skill that would be in demand in the course of a war occurring on his home planet of Tonen, and your character's assistance was requested. A contract was presented, and your character was enlisted for a one to three month period to start, with options to extend.
If you're applying here, your character agreed. Whether it was due to blackmail, coercion, the "you're going to die anyway if you stay here" card, or your character's generous nature, they agreed and signed the contract.