President Donald Trump’s pronouncement that he would be pulling troops out of Syria “very soon” has laid bare a major source of tension between the president and his generals.

Mr Trump has made winning on the battlefields of Iraq, Syria and Afghanistan a central tenet of his foreign policy and tough guy identity. But Mr Trump and the military hold frequently opposing ideas about exactly what winning means.

Those differences have played out in heated Situation Room debates over virtually every spot on the globe where US troops are engaged in combat, said senior administration officials. And they contributed to the dismissal last month of lieutenant general H R McMaster who as national security adviser had pressed the president against his instincts to support an open-ended commitment of US forces to Afghanistan.

Mr Trump’s words, both in public and private, describe a view that wars should be brutal and swift, waged with overwhelming firepower and in some cases, with little regard for civilian casualties. Victory over America’s enemies for the president is often a matter of bombing “the s*** out of them,” as he said on the campaign trail.

He returned to the theme this week. “We’re knocking the hell” out of Isis, Mr Trump said at a rally in Ohio last month. The boast was a predicate to the president insisting that US troops would be “coming out of Syria real soon”.

For America’s generals, more than 17 years of combat have served as a lesson in the limits of overwhelming force to end wars fuelled by sectarian feuds, unreliable allies and persistent government corruption. “Victory is sort [of] an elusive concept in that part of the world,” said lieutenant general Sean MacFarland, who led troops over five tours of Iraq and Afghanistan. “Anyone who goes in and tries to achieve a decisive victory is going to come away disappointed.”

Defence secretary Jim Mattis echoed that point in late November when he outlined an expanded role for US forces in preventing the return of Isis or a group like it in Syria. “You need to do something about this mess now,” he told reporters. “Not just, you know, fight the military part of it and then say, ‘good luck on the rest of it’.”

His remarks reflected a broader Pentagon consensus: in the absence of a clear outcome, winning for much of the US military’s top brass has come to be synonymous with staying put. These days, senior officers talk about “infinite war”.

“It’s not losing,” explained Air Force general Mike Holmes in a speech earlier this year. “It’s staying in the game and ... pursuing your objectives.”

The army recently rewrote its primary warfighting doctrine to account for the long stretch of fighting without victory since 9/11. “The win was too absolute,” said lieutenant general Michael Lundy of the old document. “We concluded winning is more of a continuum.”

The tension between the White House and the military over how and when to end America’s wars is not entirely new. To the frustration of his generals, president Barack Obama announced plans in 2014 to pull all US combat forces out of Afghanistan by the end of his presidency.” Americans have learned that it’s harder to end wars than it is to begin them,” he said. “Yet this is how wars end in the 21st century.”

The decision drew heavy criticism from Republican lawmakers and in 2016, with the Taliban expanding across Afghanistan, Mr Obama decided to leave about 8,000 American troops in place.

Mr Trump came to office promising to give the Pentagon a free hand to unleash the full force of US firepower. His impatience was evident on his first full day in office when he visited the CIA and was ushered up to the agency’s drone operations floor.

There agency officials showed him a feed from Syria, where Obama-era rules limited the agency to surveillance flights – part of a broader push by the previous administration to return the CIA to its core espionage mission and shift the job of killing terrorists to the military.

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Mr Trump urged the CIA to start arming its drones in Syria. “If you can do it in 10 days, get it done,” he said, according to two former officials familiar with the meeting.

Later, when the agency’s head of drone operations explained that the CIA had developed special munitions to limit civilian casualties, the president seemed unimpressed. Watching a previously recorded strike in which the agency held off on firing until the target had wandered away from a house with his family inside, Mr Trump asked, “Why did you wait?” one participant in the meeting recalled.

On the campaign trail, Mr Trump often said he would “take out” the families of terrorists.

Since taking office, Mr Trump has boosted airstrikes in Iraq and Syria, a key element in the military’s campaign to help its proxies rout Isis from its strongholds. “We’ve had tremendous military success against Isis,” Mr Trump said earlier this week. “It’s close to 100 per cent.”

But the attacks haven’t addressed the sectarian rivalries that created Isis. In some instances they have inadvertently allowed forces allied with Syrian dictator Bashar al-Assad and Iranian-backed militias to extend their influence. For many in the military and Congress, the 100 per cent defeat of Isis hardly feels like a victory.

“Who is winning in Syria?” senator Lindsey Graham, asked the commander of US forces in the Middle East last month when he appeared before lawmakers.

“Well, again, from – from the – from a civil war standpoint it would appear that the regime is ascendant,” general Joseph Votel stammered in reply.

“Is our policy still that Assad must go?” Mr Graham continued.

Mr Votel indicated that he wasn’t sure.

“Well, if you don’t know, I doubt if anybody knows, because this is your job,” an exasperated Mr Graham said.

The exchange offered a rare window into the military’s frustration. And it called to mind Mr McMaster’s oft-repeated insistence before entering the White House that simply targeting enemies was not a war-winning strategy, but the combat equivalent of, he said, “George Costanza in Seinfeld, ‘leave on an up note’ – just go in, do a lot of damage and leave.”

A similar quandary for Mr Trump has played out in Afghanistan, where US airstrikes have increased sevenfold to rates not seen since the earliest days of the war. The bombing there has arrested the Afghan government’s battlefield losses, but so far seems unlikely to alter the course of the war.

The problem isn’t a lack of military firepower, but the weak Afghan government, the persistence of safe havens in Pakistan and a Taliban movement that is fighting for its villages on terrain it knows intimately. The majority of Taliban fighters are killed within five miles of their home, US officials said.

“As we learned so painfully in Iraq, defeat has meaning only in the eyes of the defeated,” said Ryan Crocker, who served as ambassador in Iraq and Afghanistan. “The Taliban is not feeling defeated. The opposite.”

Decades earlier, Mr Crocker’s father flew a B-17 bomber as part of the American armada that reduced the German city of Dresden to scorched rubble. “That’s how you get people to feel defeated,” Mr Crocker said, “and no sane person would argue for doing it again.”

In the absence of a campaign of annihilation, the struggle to define victory in America’s never-ending wars has spanned three administrations.

Near the end of president George W Bush’s administration, Eliot Cohen recalled journeying to the basement of the Pentagon where a senior intelligence officer presented him with binders full of data that he and his staff had compiled to track US progress in Afghanistan for the defence secretary, the chairman of the Joint Chiefs and others.

“Are we winning?” Mr Cohen, then a top aide to the secretary of state, recalled asking.

The colonel looked down at the mountain of reports. “I have no idea, sir,” he replied.

Now in Kabul, senior US officials track more than 700 bench marks designed to capture the progress of the Afghan government and its security forces. US officials said the Afghans have hit 97 per cent of these goals this year. More quietly they often debate whether they are even tracking the right things.

“Are these the metrics that put you on the trajectory to winning?” one senior military adviser to the four-star commander in Kabul recalled asking over the course of 2016 and 2017. “How will you even know when you get there? God it is hard.”

Last year, military commanders came up with yet another measurement to help define victory in a war starting its 17th year. A classified study in Kabul concluded that once the Afghan government controlled 80 per cent of the population, the insurgency would be rendered “irrelevant,” said general John Nicholson, the commander of US forces in Afghanistan.

“We think if we get to about 80 per cent or more we start to reach a tipping point,” the general added.

The finding was based on a survey of insurgencies in India, Colombia, Angola, Burundi and Chad. Some have questioned the science behind the work. A similar study of 71 insurgencies by RAND Corporation, a federally funded think tank, played down the importance of holding terrain and concluded that insurgents who retain external support were nearly impossible to defeat. The Taliban continues to find sanctuary in Pakistan.

Other critics have questioned the United States’ ability to even measure control in a place such as Afghanistan, where the insurgency is mostly rural and the government’s reach has never really extended beyond major Afghan cities.

“Because the Afghan war is so difficult to wrap your head around – so insoluble – there’s a profound urge to look at things you can measure,” Mr Cohen said. “That one or two sentence summary that a distracted president with other priorities can understand.”

One answer, said veterans of Iraq and Afghanistan, is simply to set aside the whole notion of winning in favour of something else.

“What does it even mean in the 21st century?” Mr Crocker asked. “I don’t know.”

He paused and searched his mind for a suitable alternative. “I don’t think coping is one the military would be likely to embrace.”