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KOBANI, Syria — The secret military base here is the epicenter of a four-year-old U.S.-led war against the Islamic State that American commanders say has succeeded in killing nearly 65,000 fighters.

But just as the terrorist group looks to be on the brink of defeat, senior officials worry that their efforts will be wasted.

Some U.S. commanders say what they perceive as a lack of guidance from the White House — which sent Secretary of State Rex Tillerson and National Security Adviser H.R. McMaster packing in a 9-day span — is threatening their mission to destroy ISIS. Cracks are showing in America's alliance with the Kurds of northern Syria, who question whether they can rely on the U.S. under President Donald Trump.

"We’re on the two-yard line. We could literally fall into the end zone. We’re that close to total victory, to wiping out the ISIS caliphate in Syria,” one U.S. special forces commander told NBC News. “We’re that close and now it’s coming apart.”

More than a half-dozen senior officials interviewed by NBC News shared the commander’s views.

The hours of interviews revealed a profound sense of frustration, bordering on anger. NBC News agreed not to reveal the officials’ identities because they were not authorized to discuss U.S. policy in Syria publicly.

There are roughly 2,000 U.S. troops in the country, and NBC News was granted extensive access to the anti-ISIS mission there.

Multiple military officials expressed frustration that they relay their concerns in memos, but find the White House unfocused or not fully aware of the nuances of the war on ISIS in Syria.

The White House did not respond to a request for comment.

Ten weeks ago, U.S. military officials were confident that ISIS would be defeated within 90 days. But that optimistic prediction was made before Turkey launched an offensive into the Kurdish enclave of Afrin in northwest Syria in late January — a move that has further scrambled the geopolitics of the region.

Turkey’s incursion prompted Kurdish fighters in the eastern part of the country, where they had been fighting ISIS alongside U.S. special forces, to redeploy westward. With its principal ally now under attack by Turkey and distracted, the U.S. military announced an “operational pause” to its anti-ISIS fight in late March.

The concern is that ISIS may now reclaim some of its lost territory, or that fighters allied with Syrian President Bashar al-Assad may take advantage of U.S. forces’ weakened defensive position. A recent attack on a U.S. base near Deir ez-Zor by pro-regime fighters and Russian mercenaries underscored that risk.

Four years ago, ISIS swept across Syria and Iraq, capturing a territory the size of a small European country with oil fields and a population of roughly eight million.

The group's caliphate became a base for murderous extremists to plot terrorist attacks abroad. It was an execution ground for hostages, including Americans, and a training ground for thousands of foreign fighters.

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The counter-ISIS mission was somewhat straightforward in Iraq. American troops trained and re-equipped the Iraqi army so that with significant U.S. assistance, local forces eventually pushed ISIS out.

Things were for more complicated in Syria where a civil war has now entered its seventh year.

The U.S. has experimented with a variety of policies, supporting a myriad of fighters who have nearly all proven to be unreliable; the opposition groups argue that Washington never gave them enough support to succeed.

But one group has exceeded expectations: the Kurds.

“I'm very impressed. These people stood up without any international support," said Gen. Jonathan Braga, the operations director of Operation Inherent Resolve, the official name for the U.S.-led war on ISIS. "I was inspired and I was impressed.”

Gen. Jonathan Braga NBC News

U.S. military officials explained that the Kurds, unlike most of the Arab militias the U.S. had been supporting, kept their promises and communicated well with American forces.

“It’s the partnership we wished for, but never had,” said one commander who is in daily contact with the Kurdish-led forces.

Culturally and organizationally, the Kurds were different too. They could account for every weapon they were given, and they weren’t religious extremists. Women fight in near-equal numbers to men.

“Women fighting formations — I mean not in a support role, not in a logistics role — on the front line killing ISIS. That's a pretty amazing story to me," Braga said. “That's the commitment level. They trusted our first forces on the ground and we trusted them.”

U.S. special forces and their Kurdish allies wrote the rulebook as they went.

One U.S. commander recounted how they marched sheep over terrain to see if it was mined, and rolled tires down streets to test for improvised explosives. The initial 50 American special forces grew to the current 2,000.