When discussing Lady Rose's parents' marital situation, Lady Mary and Lord Grantham allude to the Marlboroughs' divorce, whose marriage resembled much Lord and Lady Grantham's. The duchess, née Consuelo Vanderbilt, was a rich American heiress who married the Duke in order to get a socially advantageous position. Their divorce in 1921, followed by an ecclesiastic annulment in 1926, was a major scandal.
When Violet mentions she attended the wedding of the Earl of Rosebury and Hannah Rothschild, in 1878, this event truly happened. Hannah Primrose (later Countess of Rosebury) inherited her father's fortune in 1874, making her the richest woman in Britain.
Lord Grantham and Shrimpy talks about Amritsar, where the Jallianwala Baug Massacre took place, in which the acting Brigadier General Reginald Dyre opened fire on unarmed Indian civilians killing at least 379 in a gathering is referred as a mad/crazy act of violence.
The poem that Carson is reading at the memorial is "For the Fallen," written by Laurence Binyon in 1914. The third and fourth stanzas of the poem are known as the Ode of Remembrance, and are often recited at Remembrance Day services.
O'Brien absconded from Downton Abbey in the middle of the night, to work for Lady Flintshire in India. Lord & Lady Flintshire return to England without servants. O'Brien then went to work for the new British governor to India.