US Acting Secretary of Defense Patrick Shanahan’s letter to his Turkish counterpart Hulusi Akar sent shock waves into Turkey’s foreign policy and security establishment and Turkey doesn’t yet know “how to reply,” a senior security source told Al-Monitor.

The official also said the letter is even “more menacing” than a letter sent from Washington to Ankara in 1964, the year that was marked by the greatest tensions between Turkey and the United States since the foundation of the modern Turkish republic.

Shanahan's letter lists concrete sanctions Turkey would face, including canceling the F-35 training program for Turkish pilots, banning Turkish officials from entering the F-35 program office as of July 31 and canceling purchases from Turkish companies as of 2020.

Turkey was not expecting such an ultimatum prior to the June 23 rerun election for mayor of Istanbul. The shock the letter created in the country can be easily seen through Ankara’s dayslong silence before deciding how to react. It has been almost a week since the Shanahan letter, sent June 6, was leaked to the media, and Ankara has just begun to speak up.