(CNN) It is the least bad of a series of pretty bad options. And it is perhaps as much about Iran as it is about Syria.

But the US announcement that it will back a border protection force for Kurds in northern Syria answers one key question: what happens once the fight against ISIS -- led and endured with huge losses by US-backed Syrian Kurds -- finally ends?

It is clear now that the US is not wholesale leaving the country, but instead planning to train up a 30,000-strong force to keep ISIS from coming back and to assist local Kurds in keeping a grasp on the land they now control northeast of the Euphrates River.

Part of this may be rooted in history: the US was accused of abandoning the Iraqi Kurds back in the '90s, and don't want to duck out again. Part of it may be about ISIS, whose militants remain scattered across the swathes of desert there, in refugee camps, or trying to cross into southern Turkey.

Part of it too may also be about Iran, an effort to curtail Tehran's growing influence in that area. Iranian-backed militias have great sway across the border in Iraq, and Iran backs the Syrian regime on the other side of the Euphrates.

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