Without some kind of new pressure, the Syrian government won’t change its treatment of ceasefires—probing, testing, and pushing, often against a single village in order to improve its tactical position. Syria’s leaders are brutal but patient; they understand it will take years to bring the country back under their control.

But perhaps the biggest question left unanswered is Iran. If this deal endures, it would help address the security concerns of Israel and Jordan, key U.S. allies, surrounding Iran. Jordan’s King Abdullah was one of the first Arab leaders to warn of the emerging, Iran-controlled Shia Crescent stretching from Iran through Iraq and Syria to Lebanon. In March, the Iran-backed Nujaba Movement militia announced it would turn to fighting Israel once the Syrian opposition was defeated. In the months and years ahead, Israel may have to contend with sizeable Iran-armed, Iraqi-Shia militias, as well as Israel’s old nemesis, Hezbollah. The new Russian-American-Jordanian deal, if it survives, would forestall forward advances closer to Israel and Jordan by these Iran-backed militias.

What Iran has to gain from this plan is unclear. Tehran, like the Syrian government itself, was not party to these negotiations, and has reacted with caution. Iran, not Russia, is Assad’s biggest source of support, and that support limits Putin’s leverage. Iran also wants Assad to retake all of Syria. Given the threats from Iran-backed Iraqi militias, there are more Iraqi-Shia fighters ready to enter the fray in Syria. Respecting the deal doesn’t seem to fit into this schema.

Iran’s critical role in Syria highlights a shortcoming in the negotiations that led to the new deal. In the Cold-War era, America and the Soviet Union, powered by their military might, could largely determine the outcome of “wars of liberation” in developing countries. In 2017 in Syria, that hasn’t worked: Either the Russians have been insincere or they lack sufficient leverage over Iran and Syria to compel any respect for ceasefires. If the new plan holds, the Trump administration wants to build upon it to negotiate further agreements, even ones backed by no-fly zones, in other parts of Syria.

But without securing acceptance from Iran and the tens of thousands of troops in Syria under its control, such a plan won’t work. The Russians brought Iran into their Kazakhstan-based Syria talks for this reason, while the Americans have limited themselves to observing, not negotiating. Until Washington accepts that diplomacy with Iran is necessary to ending the Syrian conflict, it will be left on the outside, detached from any role in building a lasting peace.

America also risks forfeiting its role in shaping the contours of a future Syria. Tillerson has said that the country must be stable in order to prevent ISIS from rising anew. But if a ceasefire process does actually work and expands, it will produce a Syria dominated by different factions: the Assad government, Syrian Kurds, and Syrian Arabs—a de facto divided Syria (partition is a dirty word in the Middle East). Such a Syria, surrounded by meddlesome neighbors, would be unstable, and mired in endless peace talks, like the moribund Geneva peace talks. The Assad regime, backed by Iran, would continue nibbling away at opposition-held territories.

In such a scenario, the resentment among the Sunni-Arab communities that facilitated the spread of ISIS and al-Qaeda in Syria since 2011 would continue to feed recruits into their next iterations. The ceasefire has positive dimensions, but on the biggest questions—preventing the further dissolution of the Middle East—it hasn’t much to offer.

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