It all began in 2014 with the siege of Kobani, a Kurdish town in Northeast Syria that was surrounded by ISIS fighters. Because the plight of the town was well-reported in the American media, Obama came under political pressure to intervene militarily to break the siege.
Until then, however, he had strenuously avoided involvement in the Syrian civil war. To be sure, he sought to avoid a quagmire, but he also was eager to avoid alienating the Iranians and the Russians.
By now, the negotiations that would lead to the Iran nuclear deal were underway. But Damascus was the close ally of both Russia and Iran, so any American intervention in Syria risked upsetting the new relationship that Obama was attempting to forge with Moscow and Tehran.
This factor is the hidden key to understanding why Team Obama gravitated to the YPG to solve its problems. The group had a long history of cordial relations with the Russians and the Iranians, and, best of all, it had no intention to topple the Assad regime. Every other group that Obama might have used to defeat ISIS had an anti-Assad agenda.