Maj Gen Nick Carter, who will take charge of 45,000 troops in six weeks, said there was an opportunity to "make a difference" in the next year.
But he said without the "luxury" of time, forces needed to show "positive trends" as quickly as possible.
"We can't be everywhere. We've... got to focus on achievable objectives."
He added: "And I think security where we know the population is living, freedom of movement on the key highways - that means the Afghan economy can start to kickstart itself, and that people can begin to take a stake in their community - is the way in which we will achieve success."
'Hotchpotch'
Maj Gen Carter's comments, in an interview with BBC security correspondent Frank Gardner, come a day after the new head of the British army said defeat for allied forces in Afghanistan would have an "intoxicating impact" on extremists around the world.
Gen Sir David Richards said the failure of a coalition of such powerful Western nations would show terrorists that "anything might be possible".
But he said without the "luxury" of time, forces needed to show "positive trends" as quickly as possible.
"We can't be everywhere. We've... got to focus on achievable objectives."
He added: "And I think security where we know the population is living, freedom of movement on the key highways - that means the Afghan economy can start to kickstart itself, and that people can begin to take a stake in their community - is the way in which we will achieve success."
'Hotchpotch'
Maj Gen Carter's comments, in an interview with BBC security correspondent Frank Gardner, come a day after the new head of the British army said defeat for allied forces in Afghanistan would have an "intoxicating impact" on extremists around the world.
Gen Sir David Richards said the failure of a coalition of such powerful Western nations would show terrorists that "anything might be possible".
And Liberal Democrat leader Nick Clegg and former leader Lord Ashdown have said the war cannot be won unless the international community changes its policies.
In a joint article for the Guardian newspaper on Friday, the pair accuse Nato of being "a hotchpotch of the committed and the half-hearted" and lacking a united strategy.
Maj Gen Carter has been running his headquarters team through final rehearsals in a high-security compound in Germany ahead of their deployment to Afghanistan.
He said: "We don't have the luxury of time, but 18 months ago there were probably 1,500 American soldiers in the south. There are now 25,000.
"And there's an awful lot more resources coming into the south as well. And with this amount of effort, I think that we do have an opportunity, during the course of the next year, to make a difference.
"But I absolutely acknowledge that time is not on our side, and we've got to show positive trends as quickly as we possibly can."
He said he was determined to seize the initiative from the Taliban and that his forces would do this by separating insurgents from the civilian population both physically and mentally.
"It's about fundamentally getting [the civilian population] to realise that the institutions that we're partnering and the... reconstruction teams... are worth supporting, rather than the insurgent," he said.
"I think that it will happen slowly, but my goodness me, there'll be a tipping point when the population will suddenly realise that it's worth being with its government institutions, rather than with the insurgent."
Talks possible
The major general's strategy will involve far closer co-operation with the Afghan security forces in a bid to raise their standards.
Maj Gen Carter admitted that roadside bombs were causing significant difficulties for Nato, but said he hoped Afghan citizens would increasingly want to tell Nato where they were hidden.
He warned that there would always be a spike in casualties before positive trends could be seen.
But, he added, the arrival in the south of more American troops would now allow Nato to hold ground and achieve progress it had been unable to make in the past.
When asked if he would talk to moderate Taliban fighters, he said "counter insurgencies are about winning an argument" and that "if we can talk to people then that may well be a quicker solution than shooting them".
In a joint article for the Guardian newspaper on Friday, the pair accuse Nato of being "a hotchpotch of the committed and the half-hearted" and lacking a united strategy.
Maj Gen Carter has been running his headquarters team through final rehearsals in a high-security compound in Germany ahead of their deployment to Afghanistan.
He said: "We don't have the luxury of time, but 18 months ago there were probably 1,500 American soldiers in the south. There are now 25,000.
"And there's an awful lot more resources coming into the south as well. And with this amount of effort, I think that we do have an opportunity, during the course of the next year, to make a difference.
"But I absolutely acknowledge that time is not on our side, and we've got to show positive trends as quickly as we possibly can."
He said he was determined to seize the initiative from the Taliban and that his forces would do this by separating insurgents from the civilian population both physically and mentally.
"It's about fundamentally getting [the civilian population] to realise that the institutions that we're partnering and the... reconstruction teams... are worth supporting, rather than the insurgent," he said.
"I think that it will happen slowly, but my goodness me, there'll be a tipping point when the population will suddenly realise that it's worth being with its government institutions, rather than with the insurgent."
Talks possible
The major general's strategy will involve far closer co-operation with the Afghan security forces in a bid to raise their standards.
Maj Gen Carter admitted that roadside bombs were causing significant difficulties for Nato, but said he hoped Afghan citizens would increasingly want to tell Nato where they were hidden.
He warned that there would always be a spike in casualties before positive trends could be seen.
But, he added, the arrival in the south of more American troops would now allow Nato to hold ground and achieve progress it had been unable to make in the past.
When asked if he would talk to moderate Taliban fighters, he said "counter insurgencies are about winning an argument" and that "if we can talk to people then that may well be a quicker solution than shooting them".
