Transitioning from campaigning to delivering on your promises is difficult for any leader, but it is proving uniquely painful for Donald J Trump.

Turning pledges into concrete action is often made harder by events but when you ride into office on grandiose promises and incendiary language it becomes diabolical.

In the heat of the campaign he called the war in Afghanistan a "total disaster", demanded a "speedy withdrawal" and slammed the "stupid leaders" increasing American commitments there.

Now he is not only going against his "instincts" because he says the consequences would be disastrous, he is leaving the door open to a political settlement with the Taliban.

He is already being reminded of his pre-election stance by the noisiest part of his base at alt-right news site Breitbart.

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And as Mr Trump acknowledged in his address today, it is not just the alt-right that are weary of war.

This conflict has cost more than a trillion dollars and the Taliban seems to have more momentum than ever.

It is also another blow to Mr Trump's isolationist America First promises and may be a tough pill for him to swallow, but the generals holding key positions in the administration (more on them later) believe it is the correct decision.

Inside accounts claim Mr Trump was particularly convinced by the legacy of not wanting to be remembered as the man who allowed terrorists to take over.

How effective it will be remains to be seen.

At its peak, the war in Afghanistan involved about 100,000 Americans. ( Reuters: Omar Sobhani )

US troop numbers peaked at about 100,000 and it is widely reported in the US an incremental increase will only bump numbers from about 10,000 to about 14,000. (Mr Trump won't reveal the number.)

After all this the Taliban is proving remarkably resilient and the President's strong words against Pakistan are unlikely to solve the problem on their own.

The other significant factor in this decision is the growing influence of the generals, some of whom clashed with the outgoing alt-right nationalist Steve Bannon.

Defence Secretary Jim Mattis reportedly played a big part in convincing the President to reverse his decision.

It would also have been deeply personal for his chief of staff John Kelly, who had the heartbreak of losing his son to the conflict in 2010.

The other member of the increasingly influential trilogy is National Security Adviser HR McMaster, who has been derided as a "globalist" by Mr Bannon.

The Afghanistan decision may be more proof of Mr Bannon's line in his exit interview that "the Trump presidency that we fought for and won is over".

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