The British Isles are made up of two countries: the Republic of Ireland and the United Kingdom (UK). The Celts first arrived in 450 BC, settling in Wales, Scotland, and Ireland. Later, the Angles, Saxons, and Vikings established small kingdoms from the 400s to 1000s AD. The Normans subsequently conquered England in the 11th century. The UK's strong economy and navy allowed it to build a vast global empire. However, independence movements in Ireland and other colonies weakened the UK in the 1900s. Today, tensions continue in Northern Ireland between the Protestant majority and Catholic minority over whether the region should remain part of the UK or join the Republic of Ireland.