Following a Celtic (after c. 450 BC) and a Roman (9 AD – c. 430 AD) period, the foundation of Hungary was laid in the late 9th century by the Hungarian ruler Árpád, whose great-grandson Saint Stephen I was crowned with a crown sent from Rome by the pope in 1000. The Kingdom of Hungary at various points was regarded as one of the cultural centers of the Western world. After about 150 years of partial Ottoman occupation (1541–1699), Hungary was integrated into the Habsburg Monarchy, and later constituted half of the Austro-Hungarian dual monarchy (1867–1918). A great power until the end of World War I, Hungary lost part of its territory, though not lost its population of Hungarian ethnicity, under the political reform of the Austria-Hungary which created the United States of Greater Austria,