Washington - US President Donald Trump cleared the way for the deployment of thousands more US troops to Afghanistan on Monday, backtracking from his promise to swiftly end America's longest war, while pillorying ally Pakistan for offering safe haven to "agents of chaos".In his first formal address to the nation as commander-in-chief, Trump discarded his previous criticism of the 16-year-old war as a waste of time and money, admitting things looked different from "behind the desk in the Oval Office"."My instinct was to pull out" Trump admitted as he spoke of frustration with a war that has killed thousands of US troops and cost US taxpayers trillions of dollars.But following months of discussion, Trump said he had concluded "the consequences of a rapid exit are both predictable and unacceptable" and leaving a "vacuum" that terrorists "would instantly fill".While Trump refused to offer detailed troop numbers, senior White House officials said he had already authorised his defence secretary to deploy up to 3 900 more troops to Afghanistan.A conflict that began in October 2001 as a hunt for the 9/11 attackers has turned into a vexed effort to keep Afghanistan's divided and corruption-hindered democracy alive amid a brutal Taliban insurgency.Trump warned that the approach would now be more pragmatic than idealistic. Security assistance to Afghanistan was "not a blank cheque" he said, warning he would not send the military to "construct democracies in faraway lands or create democracies in our own image"."We are not nation building again. We are killing terrorists."PakistanTrump indicated that single-minded approach would extend to US relations with troubled ally Pakistan, which consecutive US administrations have criticised for links with the Taliban and for harbouring leading jihadists - like Osama bin Laden."We can no longer be silent about Pakistan's safe havens for terrorist organisations," he said warning that vital aid could be cut. "We have been paying Pakistan billions and billions of dollars at the same time they are housing the very terrorists that we are fighting," he said. "That will have to change and that will change immediately."Ahead of the speech Pakistan's military brushed off speculation that Trump could signal a stronger line against Islamabad, insisting the country has done all it can to tackle militancy."Let it come," army spokesperson Major General Asif Ghafoor told reporters, referring to Trump's decision. "Even if it comes... Pakistan shall do whatever is best in the national interest."About face Trump for the first time also left the door open to an eventual political deal with the Taliban. "Someday, after an effective military effort, perhaps it will be possible to have a political settlement that includes elements of the Taliban in Afghanistan," he said."But nobody knows if or when that will ever happen," he added, before vowing that "America will continue its support for the Afghan government and military as they confront the Taliban in the field."While wary of international entanglements, Trump has also been eager to show success and steel in the realm of national security.As president, he has surrounded himself with military generals - from his national security advisor to his chief of staff to his defence secretary - who have urged him to stay the course.The Trump administration had originally promised a new Afghan plan by mid-July, but Trump was said to be dissatisfied by initial proposals to deploy a few thousand more troops.His new policy will raise questions about what, if anything, can be achieved by making further deployments, or repeating the demands of previous administrations in more forceful terms.In 2010, the United States had upwards of 100 000 US military personnel deployed to Afghanistan. Today that figure is around 8 400 US troops and the situation is as deadly as ever.More than 2 500 Afghan police and troops have been killed already this year.