Syrian Kurdish militia members make a V-sign in February. (Mursel Coban/AP)

A Syrian Kurdish militia that was initially seen by U.S. commanders as a sideshow has emerged as the strongest U.S.-backed force against the Islamic State — forcing a hasty reevaluation of U.S. strategy after the collapse of a $500 million plan for training and equipping Syrian rebels.

U.S. military commanders are now recommending a “Syria first” strategy that relies on the Kurdish fighters and a smaller Arab force to move gradually toward the Islamic State’s capital of Raqqa, just 25 miles south of the Kurds’ forward positions. The decisive battle for Raqqa could come in the spring. The stalemated campaign in Iraq and other parts of Syria probably would be left for later.

The Kurdish fighters are drawn from groups whose initials are a confusing alphabet soup. The main force of about 25,000 fighters is called the People’s Protection Units, or YPG, from its Kurdish initials. This group has formed an alliance with about 5,000 Arab tribal fighters from Raqqa and Hasakah, in northeast Syria. The umbrella command has just been dubbed the Syrian Democratic Forces , or SDF.

Russia’s military intervention complicates the budding U.S. partnership with the Kurds and their Arab allies. Russian operatives were reportedly seen in Hasakah in the past few days, discussing a possible military alliance with the Kurds. Russian boldness in talking to the United States’ favorite fighters is another sign that Moscow is bidding for overall leadership of the anti-extremist fight — without any strong U.S. pushback.

Another complication to the Kurdish strategy is Turkey, which fears U.S. aid will embolden the YPG’s allies in the Kurdistan Workers’ Party, or PKK, which Turkey regards as a terrorist group. The Turkish foreign ministry scolded the U.S. ambassador in Ankara after the United States on Monday airdropped about 100 pallets of supplies for the Syrian Kurds and Arabs, including ammunition for machine guns, mortars and rocket-propelled grenades.

But before the new Kurdish-Arab bandwagon starts its victory parade, it’s important to assess “lessons learned” from the failure of the earlier mission to build a moderate opposition force. That force cratered this summer for several important reasons:

● U.S. vetting requirements were too strict. Each individual recruit was asked to pledge that he would fight only the Islamic State, not the regime of Syrian President Bashar al-Assad. That disqualified many thousands of recruits. With the new Kurdish-Arab force, only leaders are being vetted; 20 Arab commanders were brought to Iraqi Kurdistan in August for extensive evaluations, which they passed.

● The United States had poor intelligence about Jabhat al-Nusra, the al-Qaeda affiliate that is a dominant force in the north. The moderate rebels walked into a trap in late July because they didn’t expect to be attacked by Jabhat al-Nusra fighters. That attack should have been anticipated, and the popular group may be a bigger long-term problem for the United States than the Islamic State.

● The United States couldn’t suppress other powers — such as Turkey, Qatar and Saudi Arabia — from pursuing their own, selfish proxy wars in Syria, which crippled the moderate opposition. That problem will be even more complicated now with Russia’s military intervention.

● Training required shifting local fighters to camps in Turkey and Jordan. Many fighters were afraid to leave their families in Syrian villages that were under siege. This should be less of a problem with the Kurds and Arabs, who say they are battle-ready and are fighting on their home turf.

The U.S. alliance with the Syrian Kurds was unusual in that it was largely a result of serendipity, rather than deliberate planning. It was seen at first as a delaying action, while the United States waited for the much larger “train and equip” force that was supposedly coming.

Real life interrupted this Situation Room plan. A year ago, the Kurds asked for U.S. airstrikes to liberate the town of Kobani along the Turkish border. The Islamic State lost 3,000 to 4,000 fighters in that campaign. The Kurdish forces then swept Tell Hamis and Tell Abyad in northeast Syria, capturing more than 6,500 square miles. It was an almost accidental triumph.

The U.S. introduction to the YPG came from contacts in the Iraqi Kurdish party known as the Patriotic Union of Kurdistan, or PUK (which, to further complicate matters, has close relations with Iran). The United States coordinates its air support for the YPG through a control room in Sulaymaniyah in the Iraqi region of Kurdistan, where the PUK has its headquarters.

Looking at the sudden switch in U.S. strategy, an optimist might quote the Rolling Stones lyric about how “you can’t always get what you want,” but sometimes “you get what you need.” A pessimist would wail back: “Gimme shelter.”

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