By Gary Samore
June 18, 2015

International negotiators led by the U.S. and Iran must resolve a number of critical issues to reach a comprehensive nuclear agreement by the self-imposed deadline of June 30. But the debate in Washington has already become polarized. Supporters of the emerging final agreement assert that it shuts down Iran’s nuclear-weapons program. Opponents claim it paves the pathway for an Iranian bomb. Supporters hope that the deal will empower Iran’s pragmatic factions to pursue reform. Opponents–and U.S. allies and partners in the Middle East–fear the deal will fuel Tehran’s support for terrorism and ambition to dominate the region.

The truth is more complicated–and more unknown. Under the tentative deal, Iran’s production of plutonium–one of two kinds of fissile material needed for a nuclear weapon, along with enriched uranium–is constrained indefinitely. In addition to modifying the Arak heavy-water research reactor so that it cannot produce significant quantities of plutonium, Iran is committed to not building a reprocessing facility, which is necessary to separate plutonium from spent nuclear-reactor fuel.

The constraints on uranium enrichment are less rigorous and less permanent. Iran is required to reduce its enrichment capacity–fewer centrifuges and a smaller stockpile of low-enriched uranium–so that the breakout time to produce a bomb’s worth of weapons-grade uranium is extended from the current estimate of a few months to about a year. Iran could still operate several thousand first-generation centrifuges and research more advanced centrifuges. After 10 years, the limits on enrichment capacity begin to ease, allowing Iran to deploy more advanced centrifuges and reduce breakout time. After 15 years, the constraints are entirely lifted, though more vigorous inspections and monitoring will remain in place for 20 to 25 years.

Critics argue that the U.S. could get a better deal–fewer centrifuges, longer delays, more intrusive inspections–with tougher bargaining tactics and more sanctions. But this course of action is uncertain. We intensify sanctions, and then Iran intensifies nuclear activities. It is unknowable whether this ultimately leads to a better deal or Iran’s moving closer to a bomb. But as a practical matter, the U.S. cannot walk away from the tentative agreement that it has negotiated and expect to enlist international support for more sanctions, unless Iran reneges or balks on the bargain.

The emerging nuclear deal with Iran buys time–at least a decade, and maybe more. It does not end the threat. Assuming a nuclear agreement is successfully implemented, the U.S. must take advantage of that time to contain Iran’s regional ambitions, encourage political change in Iran–as best we can–and seek a more fundamental decision by Iran to forgo its pursuit of nuclear weapons.

Samore is the executive director for research at the Belfer Center for Science and International Affairs at Harvard’s Kennedy School of Government

Contact us at editors@time.com.

This appears in the June 29, 2015 issue of TIME.

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