He announced that cutting troop numbers was now 'the basis of our strategy'.
Mr Brown spoke as two more soldiers' bodies were flown home for burial. So far 212 soldiers have died in the conflict.
He described his strategy as ' Afghanisation' - a grim echo of how Richard Nixon described the policy in Vietnam when that conflict was turning into a quagmire that ended in humiliating defeat.
Mr Brown used the word four times in his speech to the International Institute of Strategic Studies think-tank in London.
He said: 'Ours is a four-pronged strategy for accelerating the Afghanisation of the campaign. First we will partner a growing Afghan army presence in central Helmand. Second we will strengthen the civilian-military partnership, including on policing.
'Third, we will support the governor of Helmand by strengthening district government and a more effective, cleaner government in Kabul. And fourth we will build on the success of the "wheat not heroin" initiative to wean farmers from drugs crops.'
He added: 'People ask what success in Afghanistan would look like. The answer is that we will have succeeded when our troops are coming home because the Afghans are doing the job themselves.
'The issue is how fast you can move on this. What we are saying today is that we are going to move faster. As the number of Afghans taking responsibility grows and the quality of their leadership grows, we can reduce the numbers of our forces.'
The importance of his speech was thrown into sharp relief as the bodies of two more soldiers killed there in recent days were flown back to Britain.
As Mr Brown was speaking in London, their coffins were driven from RAF Lyneham through Wootton Bassett which once again paid a silent, sombre tribute.
One of two soldiers to die on Wednesday was also named today as Lance Corporal Richard Brandon, 24, from Kidderminster.
He died in an explosion in the Babaji district in Helmand. A second soldier, from 2nd Battalion The Mercian Regiment, who died after a foot patrol has yet to be identified.
Their deaths took the number of British troops killed in the war to 212, including 41 in July and August this year.
The Prime Minister spoke out the day after Eric Joyce, a parliamentary aide to Defence Secretary Bob Ainsworth, quit saying the Government could no longer claim the conflict was a fight to combat terrorism.
Downing Street officials fear Mr Joyce's intervention may be the start of an autumn campaign to destabilise Mr Brown's leadership.
But the Prime Minister responded by insisting the war is necessary to prevent Al Qaeda terrorists from using the country as a safe haven.
He claimed 'a safe Afghanistan means a safe Britain'.
The Prime Minister also boasted that his Government had increased helicopter hours, bought armoured vehicles and sent 200 specialist forces to counter roadside bombs, 1,000 of which have now been defused by the British in Helmand.
He took to task his critics by insisting: 'We cannot walk away. At times like this we must strengthen not weaken our resolve.
The Prime Minister acknowledged that he is discussing 'proper level of our troop numbers for the next stage of the exercise' with the Chief of the Defence Staff Sir Jock Stirrup and the new head of the Army, General Sir David Richards.
But he said nothing about whether the 900 extra troops sent for the Afghan election period will remain, whether Britain will send more or whether they will insist that other Nato countries do more.
And despite his hint of an early pull-out, he refused to put a formal timetable on a withdrawal.
Mr Brown's speech was criticised by Professor Michael Clarke of the Royal United Services Institute - Britain's leading military thinktank - who said Afghanistan is only an 'indirect' terrorist threat to British soil.
'The fact is the terroristswho threaten us are not in Afghanistan. If we left they would return but they are already beginning to infiltrate in the Yemen.'
Colonel Richard Kemp, who led British troops in Afghanistan in 2003, said speeding up training can only be done with more troops.
'That can only be done to an increased timetable if you increase the number of boots on the ground and with a significant increase in resources,' he said.
An American jet blasted two fuel tankers hijacked by the Taliban in Kunduz, northern Afghanistan, killing 90 people, including dozens of civilians who had rushed to the scene to collect fuel.
German commanders ordered the pre-dawn air strike after the vehicles were seized close to their base.
Officials said a drone flew over the scene 40 minutes before the attack and determined no civilians were in the area.
Brown defends Afghan war
Rhetoric vs reality - what the PM didn't say
The reality:
Ministers failed to prepare for a switch in Taliban tactics from massed battles to roadside bombs. Mastiffs are sitting idle in Britain because the units currently serving in Afghanistan have not been trained on the new vehicles.
What he said:
Between November 2006 and April this year we increased the number of helicopter hours by 84 per cent.
The reality:
Just 23 out of Britain's 500-strong helicopter fleet are in Afghanistan because most are unsuitable for the heat and altitude there. Commanders were forced to go public to complain about a lack of helicopters to increase the urgency to get more to the front line.
What he said:
Already this year we have deployed 200 specialist counter-IED troops. We are increasing our surveillance to track the Taliban and target their bomb-makers.
The reality:
Specialists were sent only after General Sir Richard Dannatt, the former head of the Army, publicly said more troops and equipment to combat roadside bombs were on a 'shopping list' of demands he passed to Mr Brown.
http://www.dailymail.co.uk/news/article-1211026/New-blow-Brown-Defence-Secretarys-aide-resigns-Afghanistan-campaign.html