Well the turks certainly think they're connected and Ash carter, obama's man, admitted it on one occasion, I'd think it odd they weren't connected. The USA need turkey as an ally, they have an airbase there and radar systems etc... geopolitically they are of more value than the kurds.
The 'sdf' was a rebranding exercise precisely because the majority of the force is ypg kurdish militia.
from the nytimes 2018:
[copy and pasted as it's behind a paywall, but 1 article is free]
The Kurdish fighters who are battling the Islamic State jihadists in Syria are regarded by the United States as its most reliable partners there. But to Turkey, a NATO ally of the United States, these Kurds are terrorists.
The Kurdish group, known as the People’s Protection Units, or Y.P.G., is now facing an escalating battle with Turkish forces in northwestern Syria, complicating American policy.
The group has deep ties to the Kurdistan Workers’ Party, also known as the P.K.K. Both Turkey and the United States consider the P.K.K. to be a terrorist organization for its violent separatist movement inside Turkey.
While Y.P.G. leaders play down their P.K.K. ties, areas they control are festooned with photos of the imprisoned P.K.K. leader, Abdullah Ocalan, viewed by Turks the same way Americans viewed Osama bin Laden.
One thing is clear: The United States, which has relied heavily on Kurdish fighters to push the Islamic State out of northeastern Syria, has consistently understated the complexities of its alliance with the Kurds, a policy some analysts call willful ignorance.
“Obviously the U.S. chose to look the other way, out of what it deemed to be the necessity of building an alliance to quickly capture territory from Daesh,” said Noah Bonsey, a Syria analyst with the International Crisis Group, using the Arabic acronym of the Islamic State.
“The U.S. has sound reasons to continue to support the Y.P.G.,” he said, “but doing so while the P.K.K. maintains an active insurgency against its NATO ally is an unsustainable situation.”
The United States military’s official partner in Syria is a militia known as the Syrian Democratic Forces, which includes Arab and Assyrian fighters but is dominated by the Y.P.G. The Americans de-emphasize such details.
....American officials have long sought to minimize the Y.P.G.’s ties to the P.K.K., but Turkey is enraged that the United States is giving military support to a group that idealizes Mr. Ocalan, the sole inmate of an island prison in the Sea of Marmara.
Many Y.P.G. leaders speak openly of their history with the P.K.K., and Kurds from Iraq, Iran and Turkey have joined the movement in Syria.
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I don't see how I am picking one part of history to fit anything, you'd prefer the USA troops in syria to front out the advancing turkish army, to help the kurds carve out their own nation on the turkish border, when none of the surrounding countries want this to happen? The americans would have to create permanent bases there to protect the kurds and fly stuff in all the time, and do this for decades, it's not feasible.
I'm not saying the sdf/kurds haven't fought bravely and it should be recognised somehow, but also by being stubborn about having their own homeland in syria on the turkish border they have forced the usa to choose, it's realpolitik, nothing to do with making a few billion, the pentagon probably wastes 10 times the amount of money the defence industry can aquire from sales to turkey imo.
I don't know all about the problems with the kurds and turks, they both follow sunni islam, so I haven't got a clue why they dislike each other vehemently, maybe the kurds were always the poorest out of the two and mistreated over the years as peasants etc..
Leningrad lindsey was saying the other week if the turks turn on the s400 system they purchased from russia, that's it the relationship/ally thing is over, so maybe this development with the kurds relates to that, the turks agreed to mothball the s400 if the US troops withdrew?