WASHINGTON — The stunning collapse of Iraq’s army in a string of cities across the north reflects poor leadership, declining troop morale, broken equipment and a sharp decline in training since the last U.S. advisers left the country in 2011, U.S. military and intelligence officials said Thursday.

Four of Iraq’s 14 army divisions virtually abandoned their posts, stripped off their uniforms and fled when confronted by militants in cities such as Mosul and Tikrit, principally fighters aligned with the radical Islamic State of Syria, the officials said.

The divisions that collapsed were said to be made up of Sunni, Shiite and Kurdish troops. Other units of mainly Shiite troops and stationed closer to Baghdad, the Iraqi capital, were believed to be more loyal to the government of Prime Minister Nouri al-Maliki, a Shiite, and would likely put up greater resistance, according to the officials.

Still, Lt. Gen. John Bednarek, who heads the office of security cooperation at the U.S. Embassy in Baghdad, told a closed hearing of the Senate Armed Services Committee that some Iraqi soldiers who guarded the Green Zone in the capital came to work wearing civilian clothes under their military uniform, according to one senator. The implication was the troops were prepared to strip to civilian attire and flee if they came under heavy attack.

Unlimited Digital Access. $1 for 4 weeks.

“That was a surprise to everybody to have four major divisions fold as quickly as they did without even fighting,” said Sen. Joe Manchin III, D-W.Va. and a committee member.

Training the Iraqi army and other security forces was a seminal mission for the U.S. before the last U.S. troops left Iraq in 2011. Despite the expenditure of billions of dollars to defend against the Islamic militancy, that army is failing at a crucial moment.

The U.S. spent about $25 billion to train and equip Iraq’s security forces and provide installations for these forces from the start of the war until September 2012, according to a report by the special inspector general on Iraq. Iraq has spent billions of its own money since then to acquire or order F-16 fighter jets, M-1 battle tanks, Apache helicopter gunships, Hellfire missiles and other weapons.

Although Iraq’s security forces vastly outnumber the Islamic State insurgents — which total up to 10,000 fighters, the Pentagon estimates — they have a number of disadvantages, including limited air power, inadequate training and poor leadership.

Before the fall of Mosul, the Iraqi forces had logistical difficulties and were battered in their clash with Islamic extremists.

From Jan. 1 through May, six helicopters were shot down and 60 were damaged in battle, an administration official said.

In the same period, 28 M-1 tanks were damaged and five tanks sustained full armor penetration by anti-tank guided missiles. The Islamic State, the administration official added, appears to have acquired Russian anti-tank weapons in Syria.

A significant number of M-1 tanks have been hobbled by maintenance issues, the official said.

“They are crumbling,” said James Dubik, a retired U.S. lieutenant general who oversaw Iraqi training during the buildup of thousands of U.S. troops into Iraq in 2007.

“There are pockets of proficiency, but in general, they have been made fragile over the past three to four years mostly because of the government of Iraq’s policies,” Dubik said. “They’re losing confidence in themselves and in the government’s ability to win. And the government is losing confidence in them.”

The failure of Iraq’s army and security services to stand up to the Islamist threat also underscores a politicization of the army leadership under al-Maliki that has corroded the military’s effectiveness at all levels, U.S. officials said.

In one instance a few years ago, a leading Sunni general in northern Iraq whom U.S. officers lauded for his operational skills was ousted and replaced by a Shiite officer. Meantime, since the last U.S. forces left, U.S. officials said the government in Baghdad had failed to finance and maintain the training missions.

The latest results are not because of the Islamic State’s strength, “but the Iraqi security forces’ weakness,” said a former senior U.S. officer who served in Iraq. “Since the U.S. left in 2011, the training and readiness of the Iraqi security forces has plummeted precipitously.”

The retired officer said the fast-moving advance south by the militants was more a reflection of the lack of resistance by Iraqi forces than the effectiveness of the Islamic State and its confederates. “If the cops abandon a city, the criminals are going to run rampant,” the officer said.