On a general level, fraud and illegality can vitiate contractual obligations.  On the former, the pretence for invading Iraq, WMD, was clearly fraudulent.  On the latter, the manner in which some of the war was conducted clearly violated the laws of armed conflict. 

In one of the recent cases, on a successful judicial review application, Justice Barnes held that the actions the soldier was required to do clearly violated the Geneva Conventions, and that this could not be seriously challenged.  

With few exception, the war resisters that I have interacted with do not expect to be kept out of combat.  In fact, this is what limits their claims to conscientious objection in the traditional sense.  That being said, what people do object to is being forced to participate in an illegal war, and in a war that is often conducted illegally.  

Your argument that a contract binds anyone to commit violations of the law is laughable.