The liberation of Mosul from the Islamic State last week was a major victory for the Iraqi people and for the United States, which supported the operation. But our broader mission in Iraq and Syria is far from over, and tactical gains against the Islamic State will be short-lived unless we deploy the right resources and choose the right partners.

This means the U.S. must resist the temptation of trying to defeat the Islamic State on the cheap by partnering with Russia in Syria – an adversary that works against our interests in the Middle East, and indeed worldwide.

Throughout Syria's six-year civil war, Russia has abetted Iranian-backed dictator Bashar al-Assad as his regime killed hundreds of thousands of civilians, forced nearly half of all Syrians from their homes, and drove millions into neighboring countries and Europe. The Islamic State was born in Iraq, but it metastasized in Syria due to Assad's brutality, and the Islamic State declared its "caliphate" when it captured Mosul in June 2014.

The Obama administration initially criticized Russia's military intervention in Syria in September 2015, but did little to push back and ultimately hoped that Moscow's involvement would help defeat the Islamic State. Instead, Russian planes targeted U.S.-backed rebels, bolstered Assad, and enabled Iran to widen its sphere of influence within Syria – threatening Washington's Sunni Arab partners and Israel.

This is why it was a surprise when Secretary of State Rex Tillerson said the other week that American and Russian objectives in Syria were "exactly the same." Why haven't we learned from the mistakes of the last administration?

To some extent, the new administration's desire to cooperate with Russia in Syria reflects the broader tendency among policymakers to see the Syrian conflict only in terms of the Islamic State. This Islamic State-centric outlook is understandable, because the Islamic State is the most urgent terrorist threat facing America and its allies. It rallies disaffected Muslims globally to commit heinous terrorist atrocities, and its thousands of jihadis will threaten us long after its "caliphate" is destroyed.

President Trump is absolutely correct to make defeating the Islamic State a top foreign policy priority. And with the fall of Mosul and the ongoing offensive in Raqaa, the United States is inching closer towards battlefield success. But achieving a lasting victory requires sustaining our partners' gains against the Islamic State on the ground.

This objective is directly at odds with Iranian goals in Syria. Iran is a revolutionary Shiite actor pursuing hegemony in a Sunni-majority region, and seeks to widen its foothold in Syria by exploiting the vacuums that the Islamic State leaves behind. An emboldened Iran would deepen the sectarian atmosphere off of which the Islamic State feeds, reverse any gains that the U.S. and its partners make against the Islamic State, and foster radicalism for years to come.

In other words, Iran and the Islamic State are two sides of the same radical coin. Success against the Islamic State both on and off the battlefield requires constraining Iran and its proxies in Syria – an objective that Russia opposes. It also requires a comprehensive, U.S.-led strategy for a post-Assad Syria.

This is why we should be skeptical of the recently-announced ceasefire with Russia, and why the Israelis have correctly rejected it. In the past, Russia and Iran have exploited similar agreements in one part of Syria by expanding their influence elsewhere in the country, violating the agreements whenever convenient. Any ceasefire proposals that cede spheres of influence to Russia or Iran are doomed to fail.

Beyond demanding a departure from the failed policies of the past, Congress has an important role to play, and in many cases, it can lead from the front. In the House, we must vote on the Russia and Iran sanctions that the Senate passed by an overwhelming margin one month ago as soon as possible. Combined with legislation sanctioning Hezbollah for its gross violations of human rights, Congress can help crack down on Iranian terrorist proxies and, through multiple public hearings, begin a broader conversation about whether Iran is actually in compliance with the Joint Comprehensive Plan of Action as the administration recently certified.

Congress, however, will remain a passive spectator unless we reclaim our Constitutionally-derived war-making responsibilities by passing a new authorization for the use of military force to govern any lethal activities against the Islamic State, al Qaeda, and their affiliates. If U.S. forces are going to shoot down any more Syrian jets or take active measures against Iranian proxies in Syria, that should require separate authorization from Congress. A new AUMF is also an opportunity for Congress to demand clarity regarding our strategic goals in the Middle East and the resources required to achieve those goals.

Members of Congress should prepare their constituents for the challenges ahead. As a Marine veteran who served in Iraq, I share President Trump's concerns about getting deeply involved in another Middle Eastern conflict, as do many Americans. But the U.S. cannot overlook the vital interests at stake in Syria. And that means being honest with the public about the costs of keeping our country safe — rather than selling the fantasy of a partnership with Russia, which basks in our insecurity.

Rep. Mike Gallagher, R-Wis., represents Wisconsin's 8th district in Congress. You can follow him on Twitter: @RepGallagher

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