Upon joining, he will swear to uphold/defend the USC.
Hence, it will be his duty to refuse orders which violate it (that's what "defending it" means).
So if he takes part in UNdeclared (and hence UNconstitutional) wars, he will:
1) Directly violate his oath by undermining that which he swore to defend
2) Undermine the legal guarantor of our freedoms and the rule of law
3) Possibly murder/maim innocent human beings (and carry that guilt the rest of his life)
If he refuses, he'll be imprisoned.
But that's what an honorable/decent/honest person would do.
That's what a real man would do.