Why is Turkey so worried about border guards?

After the Pentagon announced Jan. 13 the creation of a 30,000-member, US-backed Syrian Border Security Force (BSF), Ankara escalated threats of an operation against the Kurdish-held city of Afrin in Syria. The BSF, which will patrol the Syrian borders with Turkey and Iraq, has added fire to the already tense relationship between Ankara and Washington.

There are two issues that are triggering Ankara. First, although Washington said they are not setting up a new army, the US-trained guards that will be serving on Turkey’s border with Syria will be from the Kurdish People's Protection Units (YPG) — the largest group within the Syrian Democratic Forces (SDF). Second, the force, which will go through a training process with the CIA, will be more institutionalized and have further opportunity for achieving legitimacy. The creation of a new force means the US-YPG partnership will not end with the collapse of the Islamic State (IS), as Ankara had hoped. In his last phone conversation with President Donald Trump, Turkish President Recep Tayyip Erdogan was led to believe that US assistance to the YPG would end. According to Turkish Foreign Minister Mevlut Cavusoglu, Trump told Erdogan, “I gave instructions. No more weapons will be delivered.”