They don't call it Sin City for nothing.

During a bender that will live in royal infamy, Prince Harry, fresh from a booze-soaked getaway with pals on Necker Island, tore through Las Vegas and ended up at a game of strip billiards -- with the shocking nude photos, taken Aug. 20, going completely global a few days later.

The 27-year-old redhead "had two things in his sights: vodka and women," fellow Vegas partier Melissa Vagner tells the new Us Weekly, out now, which has all of the jaw-dropping details on Harry's wild weekend.

Adds another woman who partied with Prince William's little brother: "He was naked for a long time at the party . . . He didn't seem to care what he did."

The cover story -- which hits newsstands Friday -- also reveals how his stern grandmother Queen Elizabeth II reacted, how it could affect his military career and how the memory of his late mother Princess Diana factors in.

Pick up the new issue today!