The US has been trying to get its various allies unified in tackling the ISIS capital city of Raqqa, but continue to fail in doing so. Today, the Kurdish YPG insisted they believe they are strong enough by themselves to capture Raqqa without any involvement from the Turkish military, reiterating their opposition to Turkey getting involved.
Turkey has long had the same position, warning the US last year that the Kurds could never be allowed to take any role in the Raqqa offensive, and that they would never accept a result in which the Kurds ended up with control over the important city.
The Kurds are closer to the city right now, so the US has been embedding with them, and sending them arms and vehicles to try to make the offensive a success. At the same time, Turkey has been courting the US for a joint US-Turkey invasion of the city which would cut the Kurds out.
In both cases, the acrimony between the two factions is liable to derail any offensive, with Turkey looking to carry out offensives against YPG territory, and the YPG doubtless ready to fight any Turkish attempt to advance through their territory to Raqqa. Before they take the ISIS capital, they must first come to terms with one another.