In 1820, Great Britain destroyed the pirate fleets of seven small emirates (city-states, really) on the south coast of the Persian Gulf, and bound them together by truces: These formed a British protectorate called the Trucial States or Trucial Oman. In 1967 Trucial Oman joined OPEC. In 1971, the Emirates adopted a provisional constitution, and renamed themselves the "United Arab Emirates".

Up until 1996, the Emirates were considered as seven independent countries (by the United States, at least). I remember looking up trade figures for various countries and seeing trading partners I had never heard of -- until I found out they were part of the UAE.

In 1996, the UAE adopted a new constitution placing a permanent capital at Abu Dhabi.
Sources:
http://www.britishempire.co.uk/maproom/trucialoman.htm
http://www.eia.doe.gov/emeu/cabs/uae.html
http://www.cultural.org.ae