Christophe Soumillon brought the Verglas colt with a withering run down the wide outside and he went away to beat Le Havre and Naaqoos comfortably.
Johnny Murtagh did not get a clear run at a crucial stage as Westphalia was engaging top gear and flashed home to claim fourth without ever really looking like winning.
In a twist to the placings, an objection was lodged by Olivier Peslier, rider of Handsome Maestro, to Davy Bonilla on Naaqoos after the latter appeared to come across him in the straight.
The stewards elected to demote Naaqoos to sixth, promoting Westphalia to third.
The Yves de Nicolay-trained Silver Frost has now won six of his nine starts and beat Westphalia last time out when they met in the Prix de Fontainebleau.
There was a strong early gallop set up front but Freddie Head’s Naaqoos still ran very keen and did not help his chances at all.
Jim Bolger’s Vocalised got worked up a little bit before going into the stalls and Kevin Manning never looked to be happy at any stage. Ultimately the winner looked in a different league as he passed the post with two lengths in hand.
Aidan O’Brien said of Westphalia: “He’s run a good race and was coming home well. It is always good to see horses finishing strongly in their races and he won’t mind going further.”
Head, speaking before the demotion of Naaqoos, said: “He was a bit keen early on when he was in front but became more relaxed once other horses took up the running.
“Halfway down the straight I thought he might do something but he just died close to the line.”
Meanwhile Luca Cumani’s Fantasia could finish only third behind a Jean-Claude Rouget-trained one-two as Elusive Wave led home Tamazirte in the Poule d’Essai des Pouliches (French 1000 Guineas).
Frankie Dettori was in the perfect position throughout on the Nell Gwyn winner as he sat second behind Elusive Wave.
He asked the Cumani filly for an effort two furlongs out but the acceleration she showed at Newmarket was lacking as Christophe Lemaire kicked again on Elusive Wave.
Tamazirte made headway late on but never looked like getting to her stablemate.
Jim Bolger’s Oh Goodness Me never got competitive at the business end and Kevin Ryan’s Undaunted Affair was always in the rear.
Cashmans bet: Epsom Derby - 7/2 Fame And Glory, Sea The Stars, 6 Crowded House, 7 Gan Amhras, 8 Rip Van Winkle, 16 Masterofthehorse
Epsom Oaks: 6 Midday, 6 Rainbow View, 7 Cuis Ghaire, 8 Leocorno, 10 Sariska, Ghanaati, Enticement