Answer the questions asked on your application. I don't remember how the questions are phrased, but if they ask you if you've been convicted you can say no. However, you are asked if you've ever been arrested or used drugs, and you will have to admit to that.
Something also to consider: Unfortunately for you, the legal technicalities that allow you to say you have no record are actually seen by many employers during a background check, especially civil service jobs. You can't plead guilty to a felony or misdemeanor and expect it to disappear. It's going to follow you the rest of your life. A good defense attorney would recognize this and work to reduce the charges rather than give you the false feeling that they never happened through some legal hocus-pocus, but sometimes that's not possible and it's too late for that now, anyway.
I don't know why you decided to use drugs when you had to at least be thinking about applying for a commission in April and had to know about the military's zero-tolerance drug policy (additionally, many private companies have a similar zero-tolerance policy for their employees). "This incident" didn't kill your dream; you killed your dream, and you significantly affected your ability to be employed after college. It's a different story when you can say this happened several years ago as a teenager and you can demonstrate that you've learned your lesson, but doing this in your young 20's as you're about to enter the workplace is significantly more detrimental to your career goals, whatever they might be. On top of that, you lack ownership of your actions when you blame "this incident" rather than yourself for your situation.
