Here’s a well-developed text on the subject:
Luffy's Haki usage at Marineford wasn't a combat victory—it was a narrative turning point. when luffy use haki in marineford
This moment is crucial because it foreshadows Luffy’s true potential. Whitebeard himself sees the resemblance to Gol D. Roger and orders his crew to protect Luffy at all costs, recognizing that the boy carries the mark of a future king. Here’s a well-developed text on the subject: Luffy's
Rayleigh, the Dark King, had been watching the war from afar. When he saw Luffy unknowingly use Haoshoku Haki, he realized Luffy had finally reached the threshold to be taught. Before Marineford, Rayleigh notes that Luffy was too green to learn Haki. Roger and orders his crew to protect Luffy
At Marineford, Luffy was a glass cannon. He had the "King's" ambition but lacked the armor and weaponry to back it up against the New World's finest.
Episode 478 / Chapter 518 The Context: Ace is moments away from execution. Luffy is surrounded by enemies and running out of stamina. He is desperate.