Saudi Arabia-led coalition forces seized the strategic southern port of Aden in Yemen from Shiite Houthi rebels, unexpectedly turning the tide of Yemen’s war in favor of Saudi Arabia, Reuters reports.

“The loss of the strategic southern port of Aden in the space of just a few days is a spectacular reversal for the Houthis, a once-obscure rural group that won national importance last year before gaining the upper hand in a four-month-old civil war,” notes the report.

“At stake in what happens next is the stability of a country that is a cockpit of rivalry between regional powers Saudi Arabia and Iran, sits on leading international shipping routes and provides a haven for al Qaeda’s boldest international wing,” it adds.

Saudi Arabia and some U.S. officials have accused Iran of providing military aid to the Houthis. The Pentagon and the U.S. intelligence community have acknowledged that Iran maintains a relationship with Shiite Houthi rebels.

Nevertheless, Secretary of Defense Ashton Carter expressed doubts earlier this week as to whether the Iranian influence over the Houthis is real, noting that it is a “complicated situation.”

“If America and Iran are now on good terms … this is a show of things to come – We are ready to defend our own turf,” Abdulkhaleq Abdulla, a political scientist in the United Arab Emirates (UAE), told Reuters, referring to an Iranian nuclear deal that could pave the way rapprochement between Washington and Tehran.

“In this zero-sum game, the balance of power in the region has swung to the Gulf states’ advantage, after this net loss for Iran in Yemen,” he added, alluding to the Saudi-led coalition’s victory over the Houthis in Aden.

Sunni-led Gulf Arab nations, including Saudi Arabia and the United Arab Emirates (UAE), deployed their own special forces to capture Aden and backed thousands of anti-Houthi troops on the ground with hundreds of airstrikes. They also trained and equipped Yemeni recruits.

Secretary of State John Kerry, in an interview with PBS NewsHour in April, said the United States was aware that Iran had been providing military supplies to the Houthis in Yemen.

“It’s just not a fact. They have been,” he said.

“We’re well aware of the support that Iran has been giving to Yemen,” he explained. “And Iran needs to recognize that the United States is not going to stand by while the region is destabilized, or while people engage, you know, in overt warfare across the lines, international boundaries and other countries.”

The U.S. intelligence community’s most recent Worldwide Threat Assessment, made public in February, linked Iran to the Houthis (also spelled “Huthi”) in Yemen.

“Huthi ascendency in Yemen has increased Iran’s influence as well,” states the assessment.

A similar assessment issued by the Pentagon’s Defense Intelligence Agency (DIA) recognized Iran’s connection to the powerful Houthi militia in Yemen.

“In Yemen, instability has increased since the Huthis, a northern Zaydi Shia group with Iranian ties, captured the Presidential Palace in mid-January and attained senior positions in nearly all key Yemeni government and security institutions,” states the DIA’s 2015 Worldwide Thereat Assessment, also released in February.

Despite the Pentagon and U.S. intelligence community’s recognition that Iran has a relationship with the Houthis, Secretary of Defense Carter questioned whether the “Iranian influence with the Houthis is real.”

He added:

We have taken steps to check Iranian for example resupply of the Houthis, and emphasized the need for a political settlement there, so I think that we and the Saudis share a concern about malign influence by Iran, but we also both share an assessment that it’s a complicated situation.

Iran denies lending support to the Houthi rebels. The Houthis themselves claim they are not receiving aid from Iran, Saudi Arabia’s primary enemy in the region.