As the Syria mission quadrupled from 500 to 2,000 US troops last year, lawmakers have complained that the page-long 2001 AUMF has been applied too broadly. In letters to lawmakers last week, officials in the Donald Trump administration confirmed that the Pentagon will not seek new war powers to fight in Syria.

“We saw the rise of [IS] and the inability of Iraqi Security Forces to address it as it was growing,” Votel said of the impact of US troops' departing Iraq in 2011. Syria’s inability to deal with the IS threat in turn has jeopardized “a much broader group of countries around the world,” he said, including US partners in Jordan, Lebanon and Turkey.

“Our legal basis for operating in Syria was largely driven by the collective self-defense of Iraq,” Votel told Rep. Tulsi Gabbard, D-Hawaii, who stoked controversy by visiting Syrian President Bashar al-Assad last year. “While we were beginning to address [IS] in Iraq, we knew that we also had to address [IS] in Syria.”

US Central Command chief Joseph Votel told the House Armed Services Committee that the 2001 Authorization for Use of Military Force (AUMF), signed by President George W. Bush to go after the plotters of the Sept. 11, 2001, terrorist attacks, remains in effect for the US fight against the Islamic State (IS) in Syria. As US troops hunt down some 2,000 remaining IS fighters in the Euphrates River Valley, the legal core of the US involvement in Syria derives from Iraq’s defense, he said.

In a January speech at Stanford University, US Secretary of State Rex Tillerson said that the open-ended presence of American units could be used to stand up to Iran, which is estimated to have as many as 125,000 troops in Syria. Votel clarified today that deterring Iran is a US objective but not a "military” one.

The line of questioning underscored congressional worries that the anti-IS effort could expand indefinitely. Though the presence of 2,000 US troops in Syria is "conditions-based," Pentagon officials told Al-Monitor that there isn't a specific list of conditions for victory.

“The logical conclusion of your answer to my question about our presence after [IS] is defeated is that the US military can be in any and every country that there was ever an [IS] presence just so there will not be an [IS] presence going forward,” Rep. Beto O’Rourke, D-Texas, told Votel. “I think that is a recipe for disaster. We will not have successful oversight or accountability or prosecution of that war because we cannot define its goals or the strategy.”

The hearing was dedicated to the US approach to terrorism and a resurgent Iran. Votel outlined a three-pillar strategy to deal with Tehran, focused on deterring Iran’s growing ballistic missile capabilities, ensuring it won’t get a nuclear weapon and rolling back Iran’s influence and ability to move weapons to Shiite militias throughout the region.

“What it took Iran 25 years to do with Hezbollah in Lebanon they’re trying to do in five years with the Houthis,” Votel said, expressing concern that Iran is getting more effective at raising proxy armies that could threaten US forces, such as the Shiite-linked Yemeni group that appears increasingly capable of threatening US forces in Saudi Arabia with ballistic missiles.

Even as the Trump administration shifts its focus to great powers such as China and Russia, the Pentagon is asking for a $2 billion increase in munitions for operations in Iraq and Syria in its fiscal year 2019 budget request to fight IS. The Pentagon also wants $290 million to allow Iraq to put in place scanners, 1,500 border guards and other equipment to secure the Qaim, Bukamal, and Fish Khabur border crossings, in part to prevent Iran from moving troops, tanks and guns at its leisure.

“The land bridge only exists if Iraq allows Iran to use it,” said Michael Knights, a researcher at the Washington Institute for Near East Policy. “A complete lack of situational awareness would be a disaster. Are we worried that 6,000 [Shiite] militias will get in their cars in the next Hezbollah-Israel war and go across? Unless we get good border guards in there, they’ll be able to do that.”

The United States is fighting in Iraq at the invitation of Prime Minister Haider al-Abadi’s government and has no authority to build permanent structures. Votel said the United States is still figuring out how to measure the effectiveness of American partners in countering Iran's influence.

“We don’t want to keep one more soldier, one more piece of equipment there than is needed,” Votel said.