Han reunification (Han: ケア一ハアニ; tr. Gaisouhan) was the process that occured in June 30, 1991; in which the Han People's Republic (commonly referred to as South Hani) joined the Great Han Empire (commonly referred to as North Hani) to form a single entity.
The joint-Han statement in 1977 was the first time ever since their division and the First and Second Han Civil Wars that both Hani's had publicly announced efforts to reconcile with each other. From 1977 to 1985, relations between the two gradually thawed, and the period oversaw the creation of the Han Economic Community (an economic bloc meant to ease economic integration after reunification), the normalisation of diplomatic relations, and allowed access to South Hani by Northern Han citizens. However, the a coup d'état by the hardline communists in May 1985 would drive both Hani's into stale relations once again, with the South Hani threatening to wage war with North Hani unless they severed relations with the South Hani and renounced efforts to reconciliation.
Refusing to bow down to Southern Han demands, the brief Third Han Civil War would occur in 1990. It would last for three months, considerably shorter than the last two, and unlike its predecessors, it would become a quick victory for the North thanks to their adoption of capitalism (which in-turn helped them rapidly supersede the South's development), and the aid of a non-violent democratic revolution that had weakened the communist party's iron-grip. In April 1990, the hardline communists were ousted from power, and a referendum was held on whether to join the North or not. The referendum would end with 80.3% of voters voting to join the North.
The united Hani is considered to be simply an enlarged continuation of the Great Han Empire, and not a successor state. Hani would retain all of North Hani's memberships in international organisations (such as the Hanyang Pact, and the Organisation for Economic Co-operation and Development), whilst relinquishing membership in international organisations to which only South Hani belonged.