John Emerich Edward Dalberg-Acton, 1st Baron Acton, 13th Marquess of Groppoli, KCVO, DL (10 January 1834 – 19 June 1902), better known as Lord Acton, was an English Catholic historian, Liberal politician, and writer. A strong advocate for individual liberty, Acton is best known for his timeless observation on the dangers of concentrated authority. In an 1887 letter to an Anglican bishop, he famously wrote, 'Power tends to corrupt, and absolute power corrupts absolutely', underscoring his belief that unchecked power poses the greatest threat to human freedom. His works consistently emphasised the importance of limiting governmental and institutional power in favour of individual rights and personal liberty.

The only son of Sir Ferdinand Dalberg-Acton, 7th Baronet, and grandson of the Neapolitan admiral and prime minister Sir John Acton, 6th Baronet (who succeeded to the baronetcy and estates held by another branch of the Acton family in Shropshire in 1791), Acton was known as Sir John Dalberg-Acton, 8th Baronet, from 1837 to 1869. His grandfather was part of a younger line of the family that had moved to France and Italy, but after the extinction of the elder branch he became the patriarch.

Acton's father, known as Richard, married Marie Louise Pelline, the only daughter of Emmerich Joseph, 1st Duc de Dalberg, a naturalised French noble of ancient German lineage who had entered the French service under Napoleon and represented Louis XVIII at the Congress of Vienna in 1814. After Sir Richard Acton's death in 1837, she became the wife of the 2nd Earl Granville (1840). Marie Louise Pelline de Dalberg was heiress of Herrnsheim in Germany. She became the mother of John Dalberg-Acton who was born in Naples.

He was raised as a Roman Catholic, and was educated at Oscott College, under the future-Cardinal Nicholas Wiseman, until 1848. He then studied privately at Edinburgh. He was denied entry to the University of Cambridge because he was a Catholic, and subsequently went to Munich where he studied at the Ludwig Maximilian University of Munich and resided in the house of Johann Joseph Ignaz von Döllinger, the theologian and forerunner of the Old Catholic Church, with whom he became lifelong friends. Döllinger inspired in him a deep love of historical research and a profound conception of its functions as a critical instrument in the study of sociopolitical liberty.

He was a master of the principal foreign languages, and began at an early age to collect a magnificent historical library, which he intended to use to compose a "History of Liberty". In politics, he was always an ardent Liberal.

During his extensive travels, Acton spent much time in the chief intellectual centres of Europe, reading the correspondence of historical personalities. Among his friends were Montalembert, Tocqueville, Fustel de Coulanges, Bluntschli, von Sybel and Ranke. In 1855, he was appointed deputy lieutenant of Shropshire. A year later, he was attached to Lord Granville's mission to Moscow as British representative at the coronation of Alexander II of Russia.

In 1859, Acton settled in England, at his country house, Aldenham, in Shropshire. He was returned to the House of Commons that same year as member for the Irish borough of Carlow and became a devoted admirer and adherent of Prime Minister William Ewart Gladstone. However, Acton was not an active MP, and his parliamentary career came to an end after the general election of 1865, when he headed the Liberal ballot for Bridgnorth near his Shropshire home. Acton nearly defeated Conservative leader Henry Whitmore, but Whitmore successfully petitioned for a scrutiny of the ballots, and thus retained his seat. After the Reform Act 1867, Acton again contested Bridgnorth, this time reduced to a single seat, in 1868 but to no avail.

Acton took a great interest in the United States, considering its federal structure an ideal guarantor of individual liberties. During the American Civil War, his sympathies lay entirely with the Confederacy, for their defence of States' Rights against a centralised government that he believed would inevitably turn tyrannical. His notes to Gladstone on the subject helped sway many in the British government to sympathise with the South. After the South's surrender, he wrote to Robert E. Lee that "I mourn for the stake which was lost at Richmond more deeply than I rejoice over that which was saved at Waterloo," adding that he "deemed that you were fighting battles for our liberty, our progress, and our civilization." Acton's stance on the Confederacy was shared by most English Catholics at the time, both liberal and Ultramontane. The editors of the Ultramontane Tablet denounced Abraham Lincoln as a dangerous radical, and John Henry Newman, when asked for his opinion on the matter, stated that slavery was not "intrinsically evil" and that the issue had to be assessed on a case-by-case basis.