Washington’s obsession with expelling Islamic State from its strongholds has won it laudable tactical victories, but addressed only the symptoms driving Sunni jihadism in Iraq and Syria, rather than the causes. The fight against ISIS has allowed the Assad regime to reestablish control in Syria, while Iran has exploited the broader power vacuum to increase its leverage in Iraq at American expense. Tehran has strategically employed state and religious resources to further its hegemony in Iraq and the Levant, now wielding considerable influence over the 20 million Sunni Arabs living between Damascus and Baghdad and laying the groundwork for future conflict.However, it is not too late for decisive American action to prevent Tehran from establishing its long-sought Iranian crescent and contiguous land corridor to the Mediterranean Sea, which would greatly enable it to threaten American regional allies.Washington must recognize that it alone has the power to interrupt Iran’s strategic objectives in Syria and protect the security of its allies, but only if it chooses to act swiftly.Bearing in mind scarce American resources and the unpopularity of foreign military deployments, US strategy must be carefully crafted to minimize the risks of escalation. Moreover, since Iran can escalate in opposition to American military assets in places such as Iraq, Syria and Afghanistan, direct military confrontation against Iranian personnel or proxy forces will do little to advance Washington’s interests. Therefore, rather than antagonize Iran militarily, the US should develop a policy that supports its three primary objectives in Syria: continuing the fight against ISIS, hindering Iranian hegemony in the Levant and avoiding the escalation of a wider conflict.While these goals may seem contradictory, one can look to what the US is already doing in eastern and southern Syria to find a solution.Since March 2016, the US military has maintained a base at al-Tanf in Southern Syria near a strategic 1,200-kilometer highway connecting Baghdad, Damascus and Amman. The Syrian route is particularly valuable because it runs through to the Syrian city of Qalamoun, through which Tehran can supply Lebanese Hezbollah.Despite the US maintaining that it is only at al-Tanf to fight ISIS, this justification is contradicted by the fact that Syrian forces have had the area isolated since mid-May. Therefore, the US is remaining at al-Tanf to both train fighters indigenous to the broader Euphrates River Valley and undermine Iranian control of the Iraqi-Syrian border. Like al-Tanf, there are two other major crossings between Iraq and Syria that are consequential for regional security – one at the northern Syrian city of al-Yaarubiyah and a second at Abu Kamal in the east. Considering that Al-Yaarubiyah falls under the jurisdiction of the US-backed Kurdish People’s Protection Units (YPG), it is currently inhospitable to Iranian military movements. However, the longevity of the US-Kurdish alliance in Syria cannot be taken for granted for two reasons. First, US ally Turkey has fervently opposed American support for the YPG, as it is the Syrian offshoot of the Kurdistan Workers’ Party (PKK) – which the US and Turkey consider a terrorist group. Second, the YPG maintains economic and military ties to the Assad regime.According to James F. Jeffrey, a former US ambassador to Turkey and Iraq, Turkey’s decision last August to intervene in Syria and disrupt a contiguous Kurdish zone across northern Syria was as much to deter Kurdish aspirations for independence as it was to push back against what Turkish President Recep Tayyip Erdogan recently decried as “Persian expansionism.”Nonetheless, American support for the Kurds was enough to persuade Iran to pursue another route for transit, now seeking to cross into Syria at Abu Kamal before traveling to the city of Deir al-Zour, both of which are currently under ISIS rule.With two border crossings already under de facto American influence, it is critical that the US beat the Iranians to Abu Kamal. Per Al-Monitor columnist Amberin Zaman, there are several routes that the US coalition could take to reach the city – two from the north and one from the south. However, the northern route from Raqqa and the southern route from al-Tanf both would bring coalition forces near Assad regime forces, respectively fighting around Deir al-Zour and stationed northeast of al-Tanf.Therefore, the US should forgo provoking the regime and encourage the Syrian Democratic Forces (SDF), a primarily Kurdish-Arab force, to immediately travel south from Hasakah in Syrian Kurdistan until it reaches the Euphrates River in Deir al-Zour province. Once there, US-backed forces must capture the city to threaten ISIS’s hold on the area and restrict Iran’s control of the Syrian border.Of course, a significant caveat to this strategy is that Iran is unlikely to take these strategic losses without a fight. Yet, as Council on Foreign Relations researcher Alexander Decina has argued, the US can mitigate this risk by capitalizing on Russia’s efforts to garner international support for de-escalation zones in Syria, where the US and its allies are significant players. With a deft hand, the US can make its diplomatic support contingent on including al-Tanf and eventually Abu Kamal in this agreement, enticing Russia with international legitimacy in return for the inclusion of areas in which Moscow does not have enduring interests.This bold objective requires prudence and balancing, especially given the recent chill in US-Russian relations, but both countries have an interest in cooperating to end the war in Syria.With so much at stake and the region watching closely, the US must act now to constrain Iran before it is too late. The writer is a graduate student at George Washington University’s Elliott School of International Affairs, where he studies national security and Middle Eastern security issues. Follow him on twitter at @AdamLammon.