Back in 2015, desperate to reach a deal with Iran to limit its nuclear program, U.S. negotiators made a fateful concession: The United Nations conventional-arms embargo on Iran, they agreed, would be lifted in five years.

The costs of that concession, one of the worst mistakes of those negotiations, are about to come due. The embargo is set to expire on Oct. 18, 2020 — and if it does, the situation in the Middle East is likely to get even worse.

The concession wasn't to Iran so much as to China and Russia, two great-power rivals that participated in the nuclear negotiations. In the 1990s, China and Russia sold Iran a variety of weapons systems, which the Iranians then reverse-engineered. By this time next year, America's two most potent geopolitical rivals will have a green light to sell advanced missiles to the world's leading state sponsor of terrorism.

It would be bad enough if Iran kept those weapons for itself. But if past is prelude, there is a good chance Iran's numerous proxies in the Middle East will benefit as well.

Last week, in little-noticed testimony before the Senate Foreign Relations Committee, the U.S. special representative for Iran, Brian Hook, shared information from newly declassified U.S. intelligence assessments.

Since mid-2017, he said, Iran has "expanded its ballistic missile activities to partners across the region." That includes Hezbollah, Palestinian terrorist groups and, as of mid-2018, Shia militias in Iraq. The new intelligence also finds that Iran has increased its support of Hezbollah by helping to expand the group's ability to produce its own rockets and missiles. Finally, Hook said, the U.S. intelligence community now believes Iran is developing "missile systems and related technology solely for export to its regional proxies."

Taken together, this information underscores not only the need to extend the United Nations arms embargo, but also the limits of the current U.S. strategy of "maximum pressure." While crippling sanctions on Iran have made it much harder for groups such as Hezbollah and Shiite militias to pay salaries, they have not put a dent in Iran's broader quest to arm those proxies with weapons capable of hitting U.S. allies. The world learned this firsthand in September, when an Iranian missile destroyed a crude oil processing facility deep inside Saudi Arabia.

Since that attack, neither the U.S. nor Saudi Arabia has responded with an overt military strike. Earlier this month an Iranian oil tanker exploded in the Red Sea, but no country has claimed credit. Meanwhile, the U.S. retreat from northeastern Syria this month will potentially give Iran and its proxies more influence inside that failed state.

This geopolitical picture, combined with the new intelligence about Iran, makes the need for extending the arms embargo on Iran all the more urgent. Behnam Ben Taleblu, a senior fellow at the Foundation for Defense of Democracies, told me Wednesday that the U.N. arms embargo makes it much easier for the U.S. and its allies to devise the legal predicate to interdict weapons shipments to and from Iran.

The real danger, though, is that both China and Russia possess technology that will make Iran's already formidable military production even better. Taleblu pointed to a Chinese and Russian cruise missile that can be disguised in a cargo ship's container. If Iran can upgrade its arsenal, he said, it would be "the greatest missile power in the Middle East."

The problem for the U.S. is that any extension of the arms embargo would require agreement from both China and Russia, either of which can veto resolutions at the U.N. Security Council. This places President Donald Trump's administration in a position similar to that of its predecessor. Between 2013 and 2015, Barack Obama's administration needed Chinese and Russian support for a final deal with Iran because it believed the crippling sanctions that compelled Iran to negotiate would be toothless otherwise. And one cost of this multilateral diplomacy was the expiration of the U.N. arms embargo.

Now it's up to Hook and Secretary of State Mike Pompeo to make the case to China and Russia to forgo weapons sales to Iran for the sake of broader Middle East stability. To say that's a long shot would be an understatement.