[2] Declaration of War's dam Tempo West was descended from Glad Rags, an Irish-bred mare who won the 1000 Guineas in 1966.

[4] In early 2012 a majority share in Declaration of War was sold to John Magnier's Coolmore Stud organisation and the colt was moved to Ireland to be trained by Aidan O'Brien at Ballydoyle.

He did not race for his new trainer until 9 September when he finished fourth behind the Dermot Weld-trained seven-year-old in the Group Three Solonaway Stakes over one mile at the Curragh.

[6] 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.

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.

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.

[10] 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.

After the Breeders' Cup Declaration of War was retired and it was announced that he would stand as a stallion at Coolmore stud in County Tipperary in 2014.

[11] Declaration of War covered 172 mares in 2014 including a sister to derby winner High Chaparral and Danetime Out the dam of Toormore and Estidhkaar.