well...THE CONSTITUTION.  That IS the law, and the single most important contract between the people and our government. It is increasingly ignored.  But when an individual wants to terminate his contract  because he feels his/her duty has been corrupted and is now doing more harm than good, they point to contracts.  

I'm not saying necessarily that a deserter is a hero.  What they are doing is standing up for what they believe.  It doesn't matter what technicalities are involved in the contract, if a required action is unjust, immoral, or illegal, an individual has the right (or obligation, really) to break the contract.  

maybe another analogy will help:  you sign a contract to teach for a year.  during the year, school policy changes to require all teacher to severely beat any student who is tardy for class.  That teacher is morally obligated to refrain from complying and terminating his/her contract.