Thousands of government officials, teachers, military, police and journalists believed to be Gulen sympathizers have been rounded up and detained in Turkey.
Erdogan was given sweeping new presidential powers following a referendum April 16, which saw the office of prime minister abolished and extra powers given to the presidency.
Relations between Ankara and EU members have been under strain in recent years — particularly over Turkish accession to the EU, which was conditional on the EU-Turkey migrant deal, under which "irregular migrants" would be returned to Turkey from Greece.
Many NGOs — including the United Nations Refugee Agency UNHCR — condemned the move as they believed Turkey was not a "safe third country" under the Geneva Convention.
German-Turkish links were tested by the arrest, February 27, of Deniz Yucel, a German-Turkish journalist working for Die Welt newspaper.
He was arrested and detained on charges of supporting a terrorist organization and inciting public violence.
The granting of asylum to NATO staff and their families is likely to cause diplomatic tensions between Berlin and Ankara.