The history of Spanish–British relations is very complicated by the political heritage of the two countries. The United Kingdom was originally created by a union of the Kingdom of Scotland and England but was later joined by Ireland. Neither the United Kingdom nor Spain had a unique constitutional ancestor.
Also, the Kingdom of Spain was initially created by a union of the kingdoms of Aragon and Castile. There have also some complicated issues by the fact that the United Kingdom and Spain were both imperial powers and are after the same land with the disputed ownership and status of Gibraltar.