The island was explored in 1768 by the French navigator Louis de Bougainville. Unlike the rest of the Solomon Islands, which became British territory, Bougainville and Buka became part of German New Guinea in 1884. Occupied by Australian forces during World War I, Bougainville was mandated to Australia by the League of Nations in 1920. During World War II the island was the last Japanese stronghold in the Solomons. It became part of Papua New Guinea in 1973, despite strong secessionist sentiment. A bloody secessionist uprising, begun in the late 1980s, persisted through much of the 1990s; in 1998 a cease-fire, monitored by Australian-led forces, went into effect. A peace accord granting Bougainville broad autonomy and promising a referendum on independence was signed in 2001. Peacekeeping forces were replaced by a smaller transition team in 2003, a constitution was adopted in 2004, and a government was elected in 2005. The autonomous government has faced challenges from a former militia group that was aligned with the Papua New Guinea government during the uprising and rebels in S Bougainville that have remained outside the peace accord, but some of the rebels signed a peace accord with the autonomous government in 2007.