Three weeks ago, a group of leaders from the opposition Free Syrian Army warned U.S. officials that a strategic city along the Iraqi border was about to fall to ISIS. It was the latest in a long series of increasingly anxious cries for help. The rebels never heard back from the Americans.

Five days ago, the predictions came true. Free Syrian Army units in the city of Der al Zour handed over their territory to the Islamic State of Iraq and al Sham, following weeks of desperate requests for help to international officials, including a direct appeal in a private meeting with U.S. Ambassador to the United Nations Samantha Power.

“[The U.S. officials] showed an understanding of the situation but there was no movement at all,” the commander of the FSA battalion near Der al Zour told The Daily Beast in an interview. “There’s no clear American position in that part of Syria. We told the Americans we are going to fight ISIS and ISIS is close to us, but they did nothing.”

Two leaders of the Free Syrian Army in eastern Syria told The Daily Beast that the moderate rebels in the area greatly outnumbered the ISIS fighters returning from Iraq in stolen American-made vehicles. But the FSA battalion, weakened from months of being under siege, did not have enough ammunition to engage ISIS in the fight. They retreated south, giving up what they controlled of Der al Zour to ISIS without ISIS having to fire a shot.

“Der al Zour was surrounded for several months by ISIS and the regime. The siege included no food and water. Our battalion was weakened,” the FSA commander said. “The FSA numbers are big, but we don’t have weapons, we don’t have ammunition, we don’t have anything.”

For several weeks prior, FSA leaders had tried to sound the alarm. If Der al Zour fell to ISIS, the extremist group would have control of the transportation routes from their stronghold in Raqqa, Syria to their new territories in northern Iraq. The area is also rich in oil and gas resources and was the last FSA stronghold along the rapidly disappearing Iraq-Syria border.

About six weeks ago, a group of representatives of the FSA and the Syrian Opposition Coalition went to Jordan, and met with the international committee coordinating aid to the FSA based there. They presented a detailed urgent request for money, food, and weapons to be sent to the FSA in and around Der al Zour. Three weeks ago, just days after Mosul fell to ISIS, a group of FSA and SOC officials met with Power in Turkey and briefed her about the impending disaster on the Syrian side of the border. But after the meeting: nothing.

Power met with a range of people impacted by the Syrian conflict as part of her trip to Turkey and Jordan, an official from the U.S. mission to the United Nations (USUN) told The Daily Beast, including senior government officials, refugees, leaders of NGOs, and representatives of the Syrian opposition. She also met with Jordan's King Abdullah, Turkish President Gul and Turkish Prime Minster Erdogan.

Power’s conversations with rebel leaders included “hearing the concerns of both FSA and the Syrian opposition that they have raised consistently with the U.S. government and others, which is that the regime and ISIS are making inroads and that the Syrian opposition needs additional assistance,” the USUN official said. “Ambassador Power told them she would share what she learned with her colleagues in Washington when she returned from her trip.”

The USUN official declined to specify if or how Power followed up on the request or why the U.S. government failed to answer their call for help. The international coordinating committee in Jordan eventually did send the FSA in Der al Zour a small amount of equipment and ammunition. But the shipment didn’t arrive until after ISIS had taken over the FSA positions. ISIS, on the other hand, spread $2 million in the area, to entice local tribes and leaders to permit their presence there.

Today, Der al Zour’s city center is divided into areas controlled by the al Qaeda-affiliated al Nusrah front, the Islamic Front, and the Syrian regime, which controls the airport and the high ground outside the city from which they shell the opposition areas relentlessly. ISIS and the regime are working together, and ISIS controls the suburbs without any fear of attacks from the Syrian Arab Army.

Sen. John McCain traveled to Turkey last month and also met with FSA and SOC leaders and heard their warnings about ISIS advances in eastern Syria. He criticized the Obama administration for not heeding the warnings before Der al Zour fell to ISIS, part of what he sees as a pattern of neglecting the needs of America’s allies inside Syria.

“It’s a manifestation of our failure to help on behalf of the Syrian people. And it’s a logical conclusion to a total absence of involvement and leadership in both Syria and Iraq,” McCain told The Daily Beast in an interview.

Power, along with several other top officials including Secretary of State John Kerry, have long been arguing for more aid to the moderate fighting rebels, but have been repeatedly thwarted by senior White House national security staffers, McCain said.

“She is trying, others are trying, but the small cadre around the President, most of whom have no national security experience, are vetoing it and they have been for years,” McCain said.

Last year, The New York Times reported that White House Chief of Staff Denis McDonough rebuffed Power’s recommendation that the U.S. increase its support to the FSA. “Denis, if you had met the rebels as frequently as I have, you would be as passionate as I am,” Power told McDonough. “Samantha, we’ll just have to agree to disagree,” McDonough responded.

Senate Armed Services Committee Chairman Carl Levin told The Daily Beast in an interview that covert assistance to FSA was increasing slowly, but should have been ramped up a long time ago.

“I have never thought that we have done enough to help the Free Syrian Army. We should have given greater support to the vetted elements of the FSA a year or two years ago,” he said. “We are now in the process of doing that, but much of that is classified.”

For supporters of the Syrian opposition in Washington, the administration’s failure to support the FSA in eastern Syria was a missed opportunity and an avoidable setback in the drive to prevent ISIS from consolidating its control over its territory.

"The warning signs about the disintegration of the Iraq-Syria border have been clear for months,” said Sasha Ghosh-Siminoff, president of the American NGO People Demand Change, which works with the Syrian opposition. “It is clear that once again the White House is behind the curve in keeping pace with the realities on the ground in Syria and again missed an opportunity to empower ideologically moderate elements of the opposition who want the responsibility of keeping ISIS from further threatening the stability of the region."

Mouaz Moustafa, executive director of the Syrian Emergency Task Force, an American NGO, said that ISIS is now preparing for a push on Aleppo, Syria’s second-largest city, where the FSA is also being squeezed by extremist and regime forces. ISIS could also make another run at Azaz, a town near Aleppo on the Syria-Turkey border.

“Azaz is very strategic because it’s a major conduit for humanitarian aid and assistance to the FSA,” he said. “If ISIS takes control of territory bordering a NATO ally, that’s not good for anybody.”

The Obama administration has requested $500 million from Congress to train and equip vetted moderated opposition groups, but that aid if approved won’t be delivered until at least next year, which could be too late.

“$500 million coming several months from now is not sufficient,” Moustafa said. “We need urgent action now against both Sunni and Shia extremists.”

Omar Abu Layla, official spokesperson for the FSA Eastern Front, told The Daily Beast in an interview that the ISIS threat is getting more, not less, serious and ISIS will continue its expansion in every direction if nothing is done.

“We warned the U.S. a long time ago about the danger of ISIS. We’ve told them that if nothing is done, the danger of ISIS will spread to other countries in the region and eventually other parts of the world, including the United States,” he said. “I want to get a clear answer. Will the U.S. support the Free Syrian Army or must we wait for a new administration in Washington?”