For the first time, U.S. special operations forces entered territory in northern Aleppo held by fighters other than the Syrian Democratic Forces on Sept. 16. They were immediately confronted in the town of al-Rai by the Free Men of the East, a rebel group that is displeased with the United States' support for the Kurdish People's Protection Units (YPG). The rebels forced the U.S. troops to temporarily withdraw from the town until Turkish-backed fighters contained the situation.

The bloodless confrontation highlighted the considerable distrust and hostility that still exist between several rebel factions and the United States. Unlike Russia, which is universally admired by the loyalists on behalf of whom it has intervened, the United States' relationship with the Syrian opposition is far more complicated, in part because of its support for the YPG and in part because of the enmity between it and some of the rebels' more extreme wings.

Though the incident in al-Rai was significant, it overshadowed a far more important development in the Syrian civil war: Turkey has finally succeeded in securing direct U.S. ground support for its operations in Syria. The 40 U.S. soldiers who entered northern Aleppo are part of a new U.S. mission known as "Operation Noble Lance." According to the Pentagon, the troops will provide the same training, advice and assistance to Turkish-backed rebels that they are currently giving to the Syrian Democratic Forces. The soldiers likely will also be used to call in U.S. airstrikes in support of rebels in northern Aleppo as they advance on the Islamic State.

This ground support will be crucial to Turkey as it faces down the greatest constraint to its military activity in the country: Russia. After the Turkish air force shot down a Russian Su-24 jet in November 2015, relations between Moscow and Ankara became icy. The Russians threatened to strike at any intervening Turkish troops in northern Syria, essentially hamstringing the Turkish military. Ankara was unwilling to enter Syria alone, and Washington was reluctant to encourage or become involved in a clash between Turkey and Russia.

The thawing of ties between Turkey and Russia this summer created an opening for Ankara to intervene in northern Aleppo. However, the relationship remains strained, and Moscow has repeatedly demanded that Ankara coordinate its actions in Syria with the government of President Bashar al Assad. Nevertheless, Turkey has sent troops across its southern border. Now that it has cleared the Islamic State from the border region and contained YPG advances westward, Ankara is poised to push even closer to the Islamic State-held city of al-Bab. The United States' backing of the Turkish proxies targeting the jihadist group will make the offensive all the easier.

At the same time, the United States' presence affords Turkey the best guarantee of protection against a potential clash with Russia. Moscow would be much less willing to threaten to bomb Turkish troops and their rebel allies if U.S. troops were embedded among them. As long as Turkey has U.S. support and avoids attacking areas where Russian troops are operating, it will be fairly safe from the Kremlin's threats.

As an added bonus for Turkey, Operation Noble Lance sends a clear message to the YPG. U.S. troops, by far the biggest advantage that the Kurdish militia can leverage, have thrown their weight behind the Turkish-led advance on al-Bab rather than a YPG-led offensive. Without control of al-Bab, the YPG will lose any hope of connecting Afrin canton with Kobani canton — the Kurdish fighters' primary objective, and one that Turkey has made its mission to stop.

Several dangers still await the Turks in Syria. The Islamic State will fight tooth and nail to hold its positions in northern Aleppo, and the YPG will continue to harass and thwart the Turkish-backed rebels as best it can without losing Washington's support. Meanwhile, the Syrian government, Iran and Russia will search for ways to limit Turkey's activities in the north without instigating a conflict with U.S. or Turkish soldiers.

Even so, the biggest obstacle to Turkey's movements in northern Aleppo — the risk of a clash with Russia — has largely been removed by the United States' assistance. Ankara boasts the strongest military force in northern Syria, and its troops are allied with many rebel groups and backed by U.S. air power. With its newfound advantage, Turkey will undoubtedly set its sights on al-Bab, enlarging its footprint in northern Aleppo and containing Kurdish expansionism in Syria.