2013: four-year-old season
Declaration of War began his third season in the Listed Heritage Stakes over one mile at Leopardstown Racecourse on 14 April and won easily by 2+1⁄2 lengths from four opponents. For his next race, the colt was sent to the United Kingdom and moved up to Group One class for the Lockinge Stakes at Newbury Racecourse. He was strongly supported in the betting and started the 5/4 favourite, but weakened in the closing stages and finished fifth, more than eleven lengths behind the winner Farhh.
Declaration of War returned to England in June for the Group One Queen Anne Stakes at Royal Ascot where he started at odds of 15/2 with the American-trained five-year-old Animal Kingdom being made the 5/4 favourite. Joseph O'Brien settled the colt just behind the leaders but was unable to obtain a clear run and switched the colt to the left in the last quarter mile. In the final furlong Declaration of War accelerated into the lead and won from Aljamaheer, Gregoran and Elusive Kate. Declaration of War was beaten in his next three races, but ran well on each occasion. He finished second to Al Kazeem in the Eclipse Stakes, third to Toronado and Dawn Approach in the Sussex Stakes and fourth to Moonlight Cloud, Olympic Glory and Intello in the Prix Jacques Le Marois.
On 21 August Declaration of War started at odds of 7/1 for the Juddmonte International Stakes over 10+1⁄2 furlongs at York Racecourse. Joseph O'Brien tracked the leaders before moving the colt forward in the straight. He took the lead inside the furlongs and won by one and a quarter lengths from Trading Leather and Al Kazeem with Toronado last of the six runners. After the race O'Brien paid tribute to the winner, saying "He has an unbelievable constitution. He's made like a big sprinter, he works with a lot of speed at home, travels very strongly and definitely he's improving. Racing is the thing which is bringing him [on], which is brilliant."
Bookmakers made him the 9/4 favourite for the Irish Champion Stakes but he was withdrawn on the day of the race due to the going. The race was won by The Fugue.
On 2 November, Declaration of War was one of two European runners in the Breeders' Cup Classic at Santa Anita Park. Before Shipping to California he was given a trial gallop on the synthetic surface at Southwell Racecourse and performed very impressively, leading O'Brien to compare him favourably to his 2000 Classic runner up Giant's Causeway. He started at odds of 6.7/1 in a strong field which included Game On Dude (the favorite), Paynter, Palace Malice, Mucho Macho Man, Fort Larned, Will Take Charge and Flat Out. Joseph O'Brien settled the colt behind the leaders, before moving up to make his challenge in the straight. In a closely contested finish he finished third in a three-way photo, beaten a nose and a head by Mucho Macho Man and Will Take Charge.
