The Indonesian archipelago has been an area of migration for thousands of years. Successive movements of people brought Buddhism, Hinduism, and Islam to the region. From the 17th through the mid-20th century the Dutch colonized Indonesia. During World War II it was occupied by Japan. In 1945, Indonesian nationalists led by Sukarno declared independence, which was recognized by the Netherlands in 1949, after three years of unavailing military action against the liberation forces. Sukarno unified the country by introducing one language, and Pancasilla'(the five pillars: belief in one God, humanism, national unity, democracy, and social justice). One of the challenges of Indonesia is the balance between a unitarian state and regional autonomy, e.g. in Aceh, North Sumatra, and Tanah Papua, in the eastern part of the archipelago. Christianity in Indonesia is comprised of two religions: Protestantism and Catholicism. Protestant churches are numerous, because many are ethnic and cover an island or part of it, or an island group. The ecumenical Communion of Churches, which aims at forming one church, has over 80 member churches. The Pentecostals and the Evangelicals have each their own group, and there is overlap, and cooperation, between the three. Indonesia is the largest Muslim nation in the world. It has a tradition of tolerance, but since 1998 there has been an upsurge of Islamic extremism, resulting in violent conflicts between Muslims and Christians that have disrupted communities in Java, the Moluccas and Central Sulawesi. Many people have been killed and properties destroyed. Communal peace has become a high priority for the Christians.