In the climactic arc of the Marineford Summit War (chapters 550–580 of the One Piece manga), Monkey D. Luffy demonstrates his latent potential as a true leader by unconsciously unleashing Haoshoku Haki (Conqueror's Haki) . This moment is pivotal, as it separates him from ordinary pirates and marks him as someone with the disposition of a king.
If you are watching the anime, this corresponds to (near the end of the episode). what chapter does luffy use conqueror's haki in marineford