The Afghanistan strategy outlined today by President Donald Trump is largely what the US military has been calling for, according to counter-insurgency expert David Kilcullen.

Key points: Trump said he had changed his mind on withdrawing troops from Afghanistan and would "attack" instead

Trump said he had changed his mind on withdrawing troops from Afghanistan and would "attack" instead The US President said Afghanistan could no longer be a "safe haven" for terrorists

The US President said Afghanistan could no longer be a "safe haven" for terrorists The number of US troops in Afghanistan has declined heavily since 2010

Mr Kilcullen said the military had prevailed in a tussle between the Pentagon and the State Department, which was looking for a greater diplomatic focus.

"It is very much the sort of strategy that the military has been asking for. Not one based on a set timetable," he told ABC News in an interview from the US.

"The military's always been very concerned about presidential decisions that are focused on troop numbers, as if the troop numbers was the same as the strategy.

"I think what we've heard here is a counter-terrorism, rather than a counter-insurgency strategy, and one that is going to be based on much more significant use of lethal force and a restrained approach to, as the President called it, nation building and economic development."

Sorry, this video has expired Trump refuses to set deadline for withdrawing from Afghanistan (Photo: Reuters)

Security specialists have long argued a clear and coherent strategy on Afghanistan was sorely lacking.

Mr Trump tried to address this, by firmly limiting the scope of America's mission, saying the US would be focused on "going in there to kill terrorists".

"What I think the President was trying to do here was to ring fence the American role and say the American military does kinetics, it fights the bad guys and kills terrorists, and the rest is Afghanistan's problem," Mr Kilcullen said.

"We will not be drawn into owning the whole problem — that is where President Trump has gone here."

Mr Kilcullen said this meant a strategy largely targeted towards training and supporting the Afghan military, some direct action against terrorist groups to "obliterate" IS and stop the resurgence of the Taliban and Al Qaeda, more pressure on Pakistan, "and then not too much more beyond that".

A graph shows the rise and fall in numbers of US troops stationed in Afghanistan. ( ABC News )

He said the strategy would indicate Mr Trump was prepared to leave the Afghans to their own devices in terms of nation building and creating a strong democracy.

"[Mr Trump] says he wants an honourable and enduring outcome that is worthy of the sacrifice that Americans have put in," Mr Kilcullen said.

"So, very clearly he is not asking for democracy promotion, he is not asking for the creation of a democratic-style or, indeed, particularly functional government in Afghanistan."

Mr Kilcullen said that was a fairly modest threshold for the US to achieve.

"I think … if you read carefully between the lines, he is actually articulating a fairly low bar in terms of success," he said.

"This is carefully shrouded in triumphant rhetoric but it is quite a modest set of strategic goals he is putting forward."

Mr Kilcullen is a bestselling author and scholar, and a former professional soldier and diplomat.

Sorry, this video has expired US opens door to increased troops in Afghanistan, Trump says (Photo: Reuters)

He is founder and chairman of Caerus Global Solutions, a strategic research and design consultancy based in Washington DC, and chairman of First Mile Geo, a geospatial analysis start-up that works with communities in disaster and conflict-affected areas to create participative maps which guide humanitarian assistance efforts.

Before founding Caerus, he served 24 years as an infantry officer in the Australian Army, then with the US State Department, where he was chief strategist in the counterterrorism bureau, senior counterinsurgency adviser to General David Petraeus in Iraq, and then senior advisor for counter-insurgency to secretary of state Condoleezza Rice.