Due to its strategic position, straddling east and west, Greece was considered a vital link. In 1952, Greece became a member of NATO. On 21st April 1967 there was a coup d’etat led by colonels of the Greek military and this state of affairs continued until July 1974 when a referendum led to the rejection of constitutional monarchy, establishing in its place the current Presidential Parliamentary Democracy. Since 1981 Greece has been a member of the European Union. Evidence of modern day Greece’s long and turbulent history can be seen all over the Greek mainland and its islands. Countless archaeological sites, museums and collections of artefacts, from the Palaeolithic period to the Roman era, exist for visitors to wonder at. The period of the Byzantine and Ottoman Empires can also be seen with the many churches, monasteries, castles and other buildings and monuments in every area of Greece that the visitor will encounter.