The US made the controversial decision to back the Marxist group back in 2014 despite protests from its long-term NATO ally Turkey.
Under the direction of former president Barack Obama, the US began to support the YPG, the Syrian wing of the PKK, which has been designated a terrorist organisation by the US, EU and Turkey.
That fateful relationship has been a source of tension between the US and Turkey, two long-standing allies and the first and second-largest armies in NATO.
The PKK has waged a three-decade armed campaign against the Turkish state, leading to tens of thousands of deaths.
The nature of the YPG-US alliance has resulted in escalating tensions between Turkey and the US, especially after 2014 when Washington sent military support to the YPG in Kobani. The strain reached a climax with Turkey's operation in Afrin to clear the city of the group.
Ankara has watched with increasing concern as the YPG has been emboldened by the US transfer of weapons, military vehicles and training given to its members.
Daesh vs YPG
Before the US allied itself with the YPG, it was allied with the Syrian rebel group the Free Syrian Army (FSA).
The US, however, switched in 2014 to supporting a group that has a totalitarian Marxist ideology, a move which would have been unthinkable given that the US fought to defeat the same worldview when it was propagated by the USSR.
The US move to support a terrorist movement which seeks the destruction of Turkey also confirmed Ankara's worst fears. Far from supporting the YPG to defeat Daesh, it could be using the group instead of as a proxy against Turkey.
A Newsweek exclusive quoting a US National Security Council official is claimed to have said: "To be honest with you, it would be better for the United States to support a Kurdish nation across Turkey, Syria and Iraq...It would be another Israel in the region."
The brazen suggestion that a long-standing NATO ally and EU candidate country could be spoken about in this way will not please many of Turkey's decision-makers and will almost certainly cut across the political divide.
It also calls into question the US understanding of Turkey's foreign policy priorities and national security goals - mainly the preservation of the lives of its citizens.
Retired American diplomats have suggested in the past the US relationship was transactional.
As Ambassador [James] Jeffrey has said, the US relationship with the YPG was a transactional one in terms of conducting combat operations against the Islamic State [Daesh], which ends with the defeat of ISIS [Daesh], said Matthew Bryza, a former US diplomat and a non-resident fellow at the Atlantic Council's Eurasia Center, when talking to TRT World earlier this year.
Turkish objections were made because the new YPG-controlled territory contains most of the country's oil fields and its two biggest dams, providing the US-backed group enormous economic sources and a means to access modern weaponry, which even the YPG's umbrella organisation, the PKK, could not imagine having during its three-decade-long history.
Turkey's announcement that it will enter Syria is in no small part driven by the need to degrade an organisation that is seeking to attack Turkey and damage its territorial integrity - something, not even the US would allow.
Source: TRT World