WASHINGTON, DC – The United States government’s initial statements on the "first-step agreement on Iran’s nuclear program” have been focused, above all, on the great deal that the US and the West have gotten. Iran has agreed to halt enrichment of uranium above 5% purity; neutralize its stockpile of uranium enriched to near 20% purity; stop building its stockpile of 3.5% enriched uranium; forswear "next generation centrifuges”; shut down its plutonium reactor; and allow extensive new inspections of its nuclear facilities. In return, Iran will get "limited, temporary, targeted, and reversible relief” from international sanctions.The agreement covers only the next six months, during which both sides will try to reach a final comprehensive agreement. For now, as President Barack Obama put it, the burden remains, from the US point of view, "on Iran to prove to the world that its nuclear program will be exclusively for peaceful purposes.”Framing the issue this way reflects the need to sell even a limited, temporary deal to a skeptical US Congress. Israel’s manifest displeasure with the entire negotiating process, which Prime Minister Binyamin Netanyahu has emphasized to anyone who will listen over the past three months, reverberates loudly among Israel’s many congressional friends.Indeed, Israel’s stance bolsters the desire of Obama’s Republican opponents to paint him as weak and naïve in negotiating with Iran, a country that still describes the US as "the great Satan.” Both Republicans and Democrats are threatening to pass a new round of tough sanctions against Iran in December. Thus, Obama must focus as much on pushing back against domestic hardliners as on taking a hard line with Iranian negotiators.This is hardly surprising. One hopes that the Iranian government’s announcement to its own people reads roughly the same, in reverse, focusing on the important concessions that Iranian negotiators have won. That includes suspension of international sanctions on Iran’s exports of oil, gold, and cars, which could yield $1.5 billion in revenue; unfreezing $4.2 billion in revenue from oil sales; and releasing tuition-assistance payments from the Iranian government to Iranian students enrolled abroad.Iranian President Hassan Rouhani needs to marshal support for the deal just as much as Obama does, above all by reducing inflation and getting his country’s economy moving again. If domestic tensions, above all within Iran’s restive middle class, ease as a result, the government will receive the credit, while the Iranian Republican Guard and other hardliners will be weakened.The West had better hope that the Iranian narrative proves true, because the political space for any meaningful diplomatic agreement – both the desire for a deal and the room to achieve it – is created at home. This is particularly true when a new government comes to power with promises of improving the economy. Rouhani can undercut hardliners who would seek to block any ultimate deal only if the Iranian population both experiences economic relief and attributes it to his administration.The true test of this interim agreement, therefore, is whether both sides can secure the domestic space to continue negotiating. The stakes have never been higher – and not only because of the very real and dangerous geopolitical consequences of an Iranian bomb.As Obama put it, "If Iran seizes this opportunity, the Iranian people will benefit from rejoining the international community, and we can begin to chip away at the mistrust between our two nations. This would provide Iran with a dignified path to forge a new beginning with the wider world based on mutual respect.”Let us imagine, just for a moment, what the Middle East and Central Asia could look like if the US and Iran could once again talk to each other. As we saw briefly after the terrorist attacks of September 11, 2001, the drug trade from Afghanistan could be sharply curtailed.Moreover, a regional agreement involving Iran, India, Pakistan, Russia, China, Turkey, the European Union, and the US would become much more likely, providing the framework for security and economic growth that diplomats from Henry Kissinger to the late Richard Holbrooke always claimed would be necessary for lasting peace in Afghanistan.Perhaps most important, a peace settlement in Syria would be much more likely – and more likely to endure – if the US could talk to Iran, which has far more leverage with President Bashar al-Assad’s regime than Russia does. After all, it was fighters from Hezbollah, Iran’s Lebanese proxy, who turned the tide of battle decisively against the opposition this past summer.Iran has long made clear that it wants to resume its historic position as a major regional – and indeed global – power, an ambition that can only grow stronger as it watches Turkey’s geopolitical stature rise. Iran and Turkey, after all, are the 17th and 18th largest countries in the world by population, respectively, with sophisticated elites and illustrious and ancient pasts.The ultimate winner in the interim agreement with Iran is the cause of diplomacy itself. US Secretary of State John Kerry, EU High Representative Catherine Ashton, and the other parties to the talks – all supported by able teams of diplomats – hammered out the deal’s details over months, staying at the table, compromising, holding firm, and managing the expectations of multiple players (including the press). The Obama administration committed itself to global leadership through civilian rather than military power. That is what it takes.Anne-Marie Slaughter, a former director of policy planning in the US State Department and a Professor of Politics and International Affairs at Princeton University.Copyright Project Syndicate