The Sandline affair was a political scandal that became one of the defining moments in the history of Papua New Guinea, and particularly the conflict in Bougainville. It brought down the government of Sir Julius Chan, and took Papua New Guinea to the verge of a military revolt. The event was named after Sandline International, a UK-based private military company.

Sir Julius Chan is a Papua New Guinean politician who served as Prime Minister of Papua New Guinea from 1980 to 1982 and from 1994 to 1997. He is Member of Parliament for New Ireland Province, having won the seat in the 2007 national election. He is also the current Governor of New Ireland Province, since 2007. On 26 May 2019, Prime Minister Peter O'Neill announced he would soon resign and that he wished for Sir Julius to succeed him. An outgoing Prime Minister does not, however, have the power to appoint his successor, and the following day O'Neill delayed his own formal resignation. He was also a leading figure in his country during the years-long Bougainville conflict.