Who said "Absolute power corrupts absolutely"?

The phrase "absolute power corrupts absolutely" was first written by John Emerich Edward Dalberg Acton, also known as Lord Acton, in 1887. The full phrase is "Power tends to corrupt and absolute power corrupts absolutely. Great men are almost always bad men."

Lord Acton is widely regarded as the most learned man of his time and has been described as the magistrate of history. Born in Italy and educated in Munich, his life's work surrounded the history of political and religious liberty. The letter Lord Acton wrote, which included the phrase about absolute power, was sent to Mandell Creighton, an English historian and prelate of the Church of England.