The Republic of the Congo Civil War, lasting from June 1997 to December 1999, was fought between partisans of two presidential candidates, which ended in an invasion of Angolan forces and installation of Denis Sassou Nguesso to power. In Congo it is commonly known as the War of 5 June (Guerre du 5 juin).
Congo's democratic progress was derailed in 1997. As presidential elections scheduled for July 1997 approached, tensions mounted between the supporters of President Pascal Lissouba and former President Col. Denis Sassou Nguesso of the Congolese Labor Party (PCT). Sassou Nguesso had ruled the country from 1979 to 1992, when the country was a Marxist-Leninist single-party state.
When on June 5, President Lissouba's government forces surrounded Sassou Nguesso's compound in Brazzaville, Sassou Nguesso ordered his militia to resist. Thus began a 4-month conflict that destroyed or damaged much of the capital. In early October 1997, Angolan troops invaded Congo on the side of Sassou. Most of Brazzaville fell to rebel and Angolan forces on October 14, 1997, and Lissouba fled; within two days the capital was under the control of forces loyal to Sassou Nguesso and Pointe-Noire fell with little resistance. Soon thereafter, Sassou Nguesso declared himself President and named a 33-member government.
Alongside the political conflict between Lissouba and Sassou Nguesso, oil was considered a crucial factor in the war, and with French interests at stake, France was seen as backing Sassou Nguesso against Lissouba.
In January 1998, the Sassou Nguesso regime held a National Forum for Reconciliation to determine the nature and duration of the transition period. The Forum, tightly controlled by the government, decided elections should be held in about 3 years, elected a transition advisory legislature, and announced that a constitutional convention would finalize a draft constitution. However, the eruption in late 1998 of fighting between Sassou Nguesso's government forces and an armed opposition disrupted the transitional return to democracy. This new violence also closed the economically vital Brazzaville–Pointe-Noire railroad; caused great destruction and loss of life in southern Brazzaville and in the Pool, Bouenza, and Niari regions; and displaced hundreds of thousands of persons. However, in November and December 1999, the government signed agreements with representatives of many, though not all, of the rebel groups. The December accord, mediated by President Omar Bongo of Gabon, called for follow-on, inclusive political negotiations between the government and the opposition.