He ran third in a minor race at the Curragh twelve days later and was then stepped up in class for the Listed Marble Hill Stakes at the same track in which he finished third to Brother Bear.

In June the colt was sent to England to contest the Group 2 Coventry Stakes at Royal Ascot but made little impact as he came home fourteenth of the eighteen runners behind Rajasinghe.

[4] Twelve days after recording his first success U S Navy Flag was sent to England for the second time for the July Stakes at Newmarket Racecourse in which he was partnered by Ryan Moore.

At the Curragh on 13 August the colt was partnered by his trainer's son Donnacha O'Brien when he was stepped up to Group 1 class to contest the Phoenix Stakes.

He attempted to make all the running as usual but was outpaced in the final furlong and finished fourth behind his stablemate Sioux Nation, beaten two and a half lengths by the winner.

[5] On 30 September at Newmarket, U S Navy Flag, ridden by Heffernan, was one of twelve colts to contest the Group 1 Middle Park Stakes over six furlongs and started at odds of 10/1.

"[9] On his final appearance of the season U S Navy Flag was sent to California to contest the Breeders' Cup Juvenile at Del Mar Racetrack on 4 November.

Racing on dirt for the first time he led for most of the way before dropping back in the last quarter mile and finishing tenth of the twelve runners behind Good Magic.

[10] The race was run on heavy going and O'Brien stated "We knew US Navy Flag would hate the ground, but we needed to get him out [he] might go to the English or French Guineas and he'll grow another leg between now and then.

[12] In the St James's Palace Stakes at Royal Ascot the colt set the pace as usual but after being headed in the straight he dropped back quickly and came home ninth of the ten runners behind Without Parole.

Blue Point started favourite while the other contenders included Eqtidaar, Limato, Sands of Mali, Sioux Nation, Redkirk Warrior (Newmarket Handicap) and Brando (Prix Maurice de Gheest).

Ridden by Moore, U S Navy Flag raced up the centre of the course and led the way throughout, keeping on well in the closing stages to win by one and three quarter lengths from Brando, with third place going to the 50/1 outsider Fleet Review.

[14] In the autumn of 2018 U S Navy Flag was sent to race in Australia but had no success as finished unplaced in The Everest, the Manikato Stakes and the VRC Sprint Classic.

[15] In the official European Classification of two-year-olds for 2017, published in January 2018, U S Navy Flag was given a rating of 122, making him the best juvenile of the season, three pounds ahead of his stablemate Saxon Warrior.