The YPG is a dominant force within the Syrian Democratic Forces. Turkey considers the YPG a terrorist group.
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WASHINGTON — A controversial Kurdish referendum on whether to seek independence from Baghdad threatens to weaken the U.S.-backed fight against the Islamic State by heightening political tensions in Iraq.

Monday’s vote in Iraq’s autonomous Kurdish region could undermine a delicate military and political balance that has been instrumental in defeating the Islamic State in battles across Iraq and Syria, analysts said.

The United States has opposed the referendum, saying it has already hurt military coordination between the Kurds and Iraq’s military in the fight against the Islamic State, also known as ISIS.

The polls have closed across Iraq’s Kurdish region, and initial results are expected Tuesday. The outcome will likely be “yes” for independence, although the vote is not binding and would not result in independence soon. 

The Kurds have long harbored for an independent state but put aside those aspirations to support Iraq’s central government in recent years.

“There was still a fundamental cooperation where it really mattered — at the Baghdad political level and with the national army,” said Michael O’Hanlon, an analyst at the Brookings Institution. “Now all that is being jeopardized.”

The Kurds have their own military forces, called the peshmerga, which have been instrumental in stopping ISIS when it swept through Iraq three years ago. At the time Iraq’s national forces were in disarray and largely melted away in the face of the onslaught.

The Kurds have continued to fight alongside Iraq’s national army as ISIS militants have been driven from the country.

Now, the Kurds have control of a number…