The problem with dancing with a bear, the late Israeli Prime Minister Yitzhak Rabin once quipped, is that you can never let go. That uncomfortable reality is one the United States will be fully familiar with right now. Skepticism is mandatory. It revolves around one key question: What’s changed in the calculation of the parties that would make this effort fundamentally different than other previously failed attempts at the same thing since 2012? Are the parties exhausted? Is Syria at a breaking point? Is Russia looking for an exit strategy? Has Iranian strategy evolved? There may well be a chance to deliver this badly needed aid and perhaps to begin securing some local ceasefires between the regime and a divided opposition. And perhaps there’s more room for U.S.-Russian military cooperation against ISIS. But more than likely the Munich agreement announced by Secretary of State John Kerry and his Russian counterpart, Sergey Lavrov, isn’t ready yet for prime time. Under these circumstances, it cannot represent a serious opening toward a durable nationwide ceasefire leading to a sustainable political transition and end state to the Syrian civil war. And here are five reasons why: First, in the best of circumstances, ceasefires are made to be broken. I have been involved in diplomatic efforts to put together a number of them in Lebanon and in the Israeli-Palestinian arena, and it’s a tough assignment. And in the Syrian context, a durable ceasefire faces galactic challenges. In Syria there are hundreds – if not thousands – of large and small militias with little centralized control or discipline. And that’s true not just for the opposition that opposes the Assad regime, but for the forces of Syrian President Bashar al-Assad, too. You have pro-Syrian Iraqi Shia groups, Hezbollah units, Iranian forces and a variety of Syrian regular and irregular military forces, not to mention special forces and intelligence units that have done much of the killing. Add to this jumble the presence of al Nusra Front – an al Qaeda affiliate not covered by the ceasefire – in many rebel areas, and you have a ready-made excuse for Russian and Syrian regime violations. How a ceasefire will be monitored and enforced Syria-wide, and what will be the cost for violations and the incentives for compliance are first order headaches. Second, the reason that the Russians and Americans have even agreed to a possible de-escalation of tensions isn’t because Moscow and Tehran have seen the wisdom of standing down, but largely because in recent months Russia and Iran have consolidated support for the regime and extended Assad’s gains on the battlefield. Russia may be somewhat sensitive to criticism that its recent bombing campaign has resulted in a humanitarian catastrophe in the Aleppo area. But the main reason for Russian cooperation is that Moscow is winning its campaign to buck up Assad and can afford a pause, both for tactical and diplomatic reasons. And for that reason, the large opposition groups may find it hard to accept a cessation of hostilities that freezes the regime’s recent gains and leaves the opposition with little justification or explanation for why they have sacrificed so much for such meager gain. Third, Russia wants to make it clear at least for now that getting humanitarian aid and a ceasefire – even localized de-escalation – means accepting the reality that the Assad regime is not part of the problem but part of the solution. What the Russians want to do with Assad in the long term is still a question. But they – and Iran, to be sure – aren’t prepared to sacrifice him for now. And that will quickly cause tension with opposition groups who won’t accept Assad as part of the endgame. The Catch-22 of stability, ceasefires, and the Assad question in Syria is that the more peace and quiet that prevail, the more legitimacy and staying power Assad may acquire. How Moscow will seek to reconcile its future influence in Syria with its support for a man and a regime that have gassed, barrel-bombed and killed scores of thousands of Syrian citizens is not at all clear. It’s quite possible that Russia envisions backing Assad and an Alawi enclave, including Damascus and the coastal areas, in perpetuity. But doing so depends on how much Moscow is prepared to invest to keep Assad afloat Fourth, the inconvenient reality on the ground in Syria is that Russians, Iranians and Hezbollah – and certainly the Assad regime – have vital interests they are prepared to protect by force and sacrifice. Washington does not. It neither has the will, the allies, nor the assets on the ground to push Assad out, or to deal a determinative blow to ISIS. U.S. allies are unreliable: Turkey cares more about stopping the Syrian Kurds than overthrowing Assad; the Saudis care more about getting rid of Assad than ISIS but also won’t do much about it; and in any event, Riyadh is focused more on the threat from Iran. The United States, on the other hand, is trying to ensure that the nuclear accord with Iran is implemented and isn’t looking for a fight with Tehran in Syria. This confusion of goals and divergent agendas of allies puts Washington in a disadvantageous position compared with Russia’s much narrower and streamlined focus. Fifth, all of this makes the key U.S. goal in Syria – weakening and defeating ISIS – even more difficult to achieve. A nationwide ceasefire and a political transition that ease Assad out would of course make that goal easier. But those are distant and elusive objectives right now. And ISIS, having jumped borders and expanded its threat further into the international arena, has taken on a new and more worrisome dimension. Perhaps the Munich agreement will lead to some de-escalation and badly needed humanitarian assistance. But it is unlikely to solve the broader problem for the Obama administration mired in a broken Syria it cannot transform or escape. Join us on Facebook.com/CNNOpinion.Read CNNOpinion’s Flipboard magazine.