Tony Blair lambasted the US withdrawal from Afghanistan in an article on his institute's website, stating that the decision to leave is "political" rather than "strategic".
"The abandonment of Afghanistan and its people is tragic, dangerous, unnecessary, not in their interests and not in ours," the former British prime minister wrote.
Blair, who was prime minister from 1997 to 2007, decided to support the United States in the Afghanistan invasion and in removing the Taliban in 2001 in the wake of the September 11 attacks.
He continued to defend the decision to invade the country in the article published on Saturday, stating: "The Taliban were given an ultimatum: yield up the al-Qaeda leadership or be removed from power so that Afghanistan could not be used for further attacks."
"They refused. We felt there was no safer alternative for our security than keeping our word," Blair wrote.
He also argued in favour of intervention and said the withdrawal from Afghanistan showed the west was not committed:
"The world is now uncertain of where the West stands because it is so obvious that the decision to withdraw from Afghanistan in this way was driven not by grand strategy but by politics," Blair wrote.
Many were surprised at how quickly the Taliban took over as the US withdrew its troops from the country and have called the situation "catastrophic."
For years, US presidents have said they would reduce or withdraw troops in a war that has been extremely unpopular among American taxpayers.
"It is a source of deep sadness for many of us across Nato, and no one wanted 20 years of sacrifice to end this way. We will do our best to the very last moment," UK defence secretary Ben Wallace wrote in an article published in the Mail on Sunday.
Criticism of August 31 deadline
US President Joe Biden had given a deadline of August 31 for withdrawing US troops, though he said that could depend on how many evacuations they can do per day.
Blair appeared to criticise that deadline, stating: "there must be no repetition of arbitrary deadlines. We have a moral obligation to keep at it until all those who need to be are evacuated."
Wallace admitted in the Mail on Sunday that if the US deadline stands, "we have no time to lose to get the majority of the people waiting out."
Meanwhile EU foreign policy chief Josep Borrell told AFP that it would be "impossible" to evacuate all of the Afghans that worked with western countries by then.