President Donald Trump is vowing to win what has seemed to be an unwinnable war.

How he plans to do so is still murky despite the months of internal deliberations that ultimately persuaded Trump to stick with a conflict he has long opposed.

In a 26-minute address to the nation on Monday, Trump alluded to more American troops deploying to Afghanistan, but refused to say how many. He said victory would be well-defined, but outlined only vague benchmarks for success, like dismantling al-Qaeda and preventing the Taliban from taking over Afghanistan. He said the US would not offer Afghanistan a “blank check,” but provided no specific timetable for the end of an American commitment that has already lasted 16 years.

Instead, Trump projected an “I got this” bravado that has become a hallmark of his presidency.

“In the end, we will win,” he declared of America's longest war.

Victory in Afghanistan has eluded Trump's predecessors: President George W. Bush, who launched the war after the 11 September 2001 attacks, and President Barack Obama, who surged US troop levels to 100,000, but ultimately failed in fulfilling his promise to bring the conflict to a close before leaving office.

As Trump now takes his turn at the helm, he faces many of the same challenges that have bedevilled those previous presidents and left some US officials deeply uncertain about whether victory is indeed possible.

Afghanistan remains one of the world's poorest countries and corruption is embedded in its politics. The Taliban is resurgent. And Afghan forces remain too weak to secure the country without American help.

“When we had 100,000 US troops in Afghanistan, we couldn't secure the whole country,” said Ben Rhodes, who served as Obama's deputy national security adviser.

Trump offered up many of the same solutions tried by his predecessors. He vowed to get tough on neighbouring Pakistan, to push for reforms in Afghanistan and to moderate ambitions. The US will not be caught in the quagmire of democracy-building abroad, he said, promising a “principled realism” focused only on US interests would guide his decisions.

Obama promised much of the same.

By simply sticking with the Afghan conflict, Trump's plan amounts to a victory for the military men increasingly filling Trump's inner circle and a stinging defeat for the nationalist supporters who saw in Trump a like-minded sceptic of US intervention in long and costly overseas conflicts. Chief among them is ousted adviser Steve Bannon, whose website Breitbart News blared criticism Monday of the establishment's approach to running he war.

After Trump's speech, one headline on the website read: “'UNLIMITED WAR.” Another said: “What Does Victory in Afghanistan Look Like? Washington Doesn't Know.”

Now Trump leads Washington and that question falls for him to answer. As a candidate, he energised millions of war-weary voters with an “America First” mantra and now faces the challenges of explaining how that message translates to US involvement in a war across the globe, likely for years to come.

In a rare moment of public self-reflection, Trump acknowledged that his position on Afghanistan had changed since taking office and sought to sway his supporters who would normally oppose a continuation of the war.

“My original instinct was to pull out,” Trump said. “But all my life I've heard that decisions are much different when you sit behind the desk in the Oval Office, in other words, when you're president of the United States.”

Trump pointed to “three fundamental conclusions” about US interests in Afghanistan — all of which appeal to patriotism and nationalistic pride.

The president said the nation needs to seek “an honourable and enduring outcome worthy of the tremendous sacrifices” made by US soldiers — a line that harkened back to promises made by Richard Nixon during the 1968 campaign to bring “an honourable end” to the war in Vietnam.

Trump also warned that a rapid exit would create a vacuum that terrorists like the Islamic State group and al-Qaeda would fill, leading to conditions similar to before the 11 September 2001 attacks. And he noted that the security threats in Afghanistan are “immense,” and made the case that it is key to protecting the US.

The US currently has about 8,400 troops in Afghanistan. Pentagon officials proposed plans to send in nearly 4,000 more to boost training and advising of the Afghan forces and bolster counter-terrorism operations against the Taliban and an Islamic State group affiliate trying to gain a foothold in the country.

Afghanistan: Between Hope and Fear Show all 16 1 /16 Afghanistan: Between Hope and Fear Afghanistan: Between Hope and Fear 2001 Afghans at the Killi Faizo refugee camp desperately reach for bags of rice being handed out to the thousands who escaped the bombardment in southern Afghanistan during Operation Enduring Freedom. (Chaman, Pakistan, December 4, 2001) Paula Bronstein Afghanistan: Between Hope and Fear 2002 Mahbooba stands against a bullet-ridden wall, waiting to be seen at a medical clinic. The seven-year-old girl suffers from leishmaniasis, a parasitical infection. (Kabul, March 1, 2002) All photos Paula Bronstein Afghanistan: Between Hope and Fear 2003 A mother and her two children look out from their cave dwelling. Many families who, fleeing the Taliban, took refuge inside caves adjacent to Bamiyan’s destroyed ancient Buddha statues now have nowhere else to live. (Bamiyan, November 19, 2003) Paula Bronstein Afghanistan: Between Hope and Fear 2007 Students recite prayers in a makeshift outdoor classroom in the Wakhan Corridor, a mountainous region in northeastern Afghanistan that extends to China and separates Tajikistan from India and Pakistan. (Northeastern Afghanistan, September 2, 2007) Paula Bronstein Afghanistan: Between Hope and Fear 2007 Bodybuilders in the 55-60 kg category square off during a regional bodybuilding competition. Many Afghan men, like others around the world, feel that a macho image of physical strength is important. (Kabul, August 6, 2007) Paula Bronstein Afghanistan: Between Hope and Fear 2008 A woman in a white burqa enjoys an afternoon with her family feeding the white pigeons at the Blue Mosque. (Mazar-e-Sharif, March 8, 2008) Paula Bronstein Afghanistan: Between Hope and Fear 2009 Addicts inject heroin while trying to keep warm inside the abandoned Russian Cultural Center, which the capital city’s addicts use as a common gathering point. Heroin is readily available, costing about one dollar a hit. (Kabul, February 9, 2009) Paula Bronstein Afghanistan: Between Hope and Fear 2009 An elderly man holds his granddaughter in their tent at a refugee camp after they were forced to flee their village, which US and NATO forces had bombed because, they claimed, it was a Taliban hideout. (Surobi, Nangarhar Province, February 7, 2009) Paula Bronstein Afghanistan: Between Hope and Fear 2009 Seven-year-old Attiullah, a patient at Mirwais Hospital, stands alongside an X ray showing the bullet that entered his back, nearly killing him. Attiullah was shot by US forces when he was caught in a crossfire as he was herding sheep. (Kandahar, October 13, 2009). Paula Bronstein Afghanistan: Between Hope and Fear 2010 US Army Sargeant Jay Kenney (right), with Task Force Destiny, helps wounded Afghan National Army soldiers exit a Blackhawk helicopter after they have been rescued in an air mission. (Kandahar, December 12, 2010) Paula Bronstein Afghanistan: Between Hope and Fear 2010 An Afghan National Army battalion marches back to barracks at the Kabul Military Training Center. (Kabul, October 4, 2010) Paula Bronstein Afghanistan: Between Hope and Fear 2014 Eid Muhammad, seventy, lives in a house with a view overlooking the hills of Kabul. He and millions of other Afghans occupy land and housing without possessing formal deeds to them. (Kabul, November 21, 2014) Paula Bronstein Afghanistan: Between Hope and Fear 2014 Razima holds her two-year-old son, Malik, while waiting for medical attention at the Boost Hospital emergency room. (Lashkar Gah, Helmand Province, June 23, 2014) Paula Bronstein Afghanistan: Between Hope and Fear 2014 Young women cheer as they attend a rally for the Afghan presidential candidate Ashraf Ghani. (Kabul, April 1, 2014) Paula Bronstein Afghanistan: Between Hope and Fear 2014 Burqa-clad women wait to vote after a polling station runs out of ballots. (Kabul, April 5, 2014) Paula Bronstein Afghanistan: Between Hope and Fear 2015 Relatives, friends, and women’s rights activists grieve at the home of Farkhunda Malikzada, who was killed by a mob in the center of Kabul. Farkhunda was violently beaten and set on fire after a local cleric accused her of burning a Qur’an. (Kabul, March 22, 2015) Paula Bronstein

To those US service members, Trump promised nothing short of success.

“The men and women who serve our nation in combat deserve a plan for victory,” he said. “They deserve the tools they need and the trust they have earned to fight and to win.”