Why did Turkey close the border with Armenia 16 years ago?

Turkey closed the border with Armenia 16 years ago because of the conflict in Azerbaijan over the disputed enclave of Nagorno-Karabakh. At the time, Armenian forces had occupied large swathes of territory outside the enclave. (4)The agreement signed in Zurich this week to establish diplomatic relations with Armenia was made possible only because the current government takes a more pragmatic and less nationalistic approach towards foreign policy than its predecessors. Nevertheless, Prime Minister Recep Tayyip Erdogan has repeatedly promised not to open the border with Armenia until its troops pull back in Azerbaijan.