The hasty maritime departure of Yemen's US.-backed president on Wednesday illustrated how completely one of the most important American counterterrorism efforts has disintegrated, leaving the country wide open for what could be a deeply destabilizing proxy war between Saudi Arabia and Iran.

Driven weeks ago from the capital by Shiite rebels, President Abed Rabbo Mansour Hadi abandoned the country, leaving on a boat from the southern port of Aden, Yemeni security officials said. His departure came after air strikes rained down on his troops, a sign that rebels held air superiority and that Hadi's calls for an international no-fly zone had been disregarded. On the ground, the rebels were advancing toward his position.

Three years ago, American officials hailed Hadi's ascension to power in a U.S.-brokered deal that ended the longtime rule of Ali Abdullah Saleh during the political upheaval of the Arab Spring. And just a few months ago, President Barack Obama was still calling Yemen a counterterrorism success story, even as the CIA warned that Iranian-backed Houthi rebels were growing restive in the north of the country.

Now, U.S. officials acknowledge their efforts against Yemen's dangerous al-Qaida affiliate are seriously hampered, with the American embassy closed and the last U.S. troops evacuated from the country over the weekend. Although the Houthis have seized control of much of the country and are avowed enemies of al-Qaida, they can't project power against the militants the way the Hadi government could with American support, officials say. Deeply anti-American, the Houthis have rejected U.S. overtures, officials say.

Al-Qaida in the Arabian Peninsula, or AQAP, is considered the terror group most dangerous to the U.S. because it successfully placed three bombs on U.S. bound airlines, although none exploded. The chaos in Yemen will give the group breathing space, American officials acknowledge.

Beyond terrorism, the latest developments in Yemen have worrisome implications for a Middle East already wracked by Sunni-Shia conflict, experts say. Sunni power Saudi Arabia bolstered its troop presence Wednesday along its border with Yemen. Although Pentagon officials said there was no sign of an imminent invasion, Saudi officials are deeply disturbed by the rise of the Shiite Houthis.

Meanwhile, the Houthis are widely seen as having links to Iran, and while those ties are not explicit as the Iranian relationship with Hezbollah in Lebanon or Shiite militias in Iraq, the U.S. government has said publicly that Iran has provided the Houthis with weapons and other support.

"This is all about Sunni vs. Shia, Saudi vs. Iran," said Michael Lewis, professor at Ohio Northern University College of Law and a former Navy fighter pilot who watches Yemen closely. The U.S., he said, "can't be a disinterested observer. Nobody's going to buy that. What we needed to do was pick a side."

But the U.S. had made no move to protect the Hadi government as the Houthis advanced, and American officials gave no indication Wednesday that their stance of neutrality had changed. Asked whether the U.S. military had considered trying to rescue Hadi, a senior American official who declined to be quoted answered: "The tinder box in Yemen is most complicated because of the geopolitics at stake. The U.S., Saudis, Iranians, Houthis, Yemenis, AQAP, ISIL, and AQ have equities in the situation and factor into any decision the U.S. makes or doesn't make."

In the past, American officials had stressed that their only military goal in Yemen was in defeating al-Qaida, and that they would not get involved in a Yemeni civil war.

"Our policy was, 'the Houthis, that's an internal problem, we're not involved. We're interested in AQ,'" said Barbara Bodine, a former U.S. ambassador to Yemen, referring to al-Qaida.

"The tragedy is that unlike Syria, which never looked like it was going to come out well, Yemen was doing very well on the transition and they could have pulled this thing out," she said. "The Yemenis have responsibility for a lot of this, but we weren't seen as really invested in the governance and economic issues that drove the Arab Spring revolution in the first place."

As late as Monday, officials insisted the U.S. was still working with Hadi's government, despite the fact that the president had been forced out of the capital and the parliament dissolved.

"There continues to be ongoing security cooperation between the United States and the national security infrastructure of the Hadi government," White House spokesman Josh Earnest said.

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Associated Press writers Lolita C. Baldor and Julie Pace contributed to this report.