The background check is pretty big picture. Keep in mind that when you enter the military, you also sign a sheet of paper that asks you whether or not you've done any other things, ranging from speeding tickets to felonies to marijuana use. I'm willing to bet that at least 50% have marked the "I've used recreational amounts of marijuana" box. They still get in and they still get a security clearance.

They're looking for felonies, histories of deceitfulness, ability to be trusted, etc. They really don't care if your neighbor doesn't like you because you had a party 5 years ago or something stupid like that. On the other hand, if your neighbor saw you holding Al Qaeda meetings and dealing cocaine...