David Cameron has said the UK must not be an "uncritical" or "slavish" ally of the US - and said recent foreign policy had lacked "humility and patience".
Mr Cameron says deployment of troops should be a 'last resort'
The Conservative leader accused the US and UK of viewing the "war on terror" in "unrealistic and simplistic" terms.
He insisted he was not a "neo conservative" on foreign policy - a rebuke to what is seen as the guiding philosophy of the Bush administration.
But he said the Tories were "passionate supporters" of the US-UK relationship.
Mr Cameron spoke out on the fifth anniversary of the 11 September attacks on the US.
He said the Conservatives were "instinctive friends" of the trans-Atlantic alliance, but argued that this did not mean being "slavish in our friendship with America".
Until recently the UK was not an uncritical ally of America, and had acted as a junior partner to the US for more than half a century.
"Churchill, though he found it difficult, was junior partner to Roosevelt; Margaret Thatcher to Ronald Reagan, John Major to George Bush Senior in the First Gulf War," he said.
"It is not an easy part to play, but these three prime ministers learned to carry it through with skill and success. I worry that we have recently lost the art.
"I fear that if we continue as at present we may combine the maximum of exposure with the minimum of real influence over decisions.
"The sooner we rediscover the right balance the better for Britain and our alliance. This is not anti-American. This is what America wants."
In his speech at the British American Project's annual JP Morgan lecture, Mr Cameron argued that US and UK foreign policy has lost "humility and patience" and said it had unintentionally "fanned the flames" of anti-Americanism.
Describing himself as a liberal conservative, rather than a neo conservative, Mr Cameron said he had backed the US-led interventions in Iraq and Afghanistan.
But he argued that trying to transform a country from tyranny to freedom "does not begin and end with regime change and the calling of elections".
"It cannot easily be imposed from outside," he said. "Liberty grows from the ground - it cannot be dropped from the air by an unmanned drone."
He argued that troops should only be deployed "as a last resort" and called for the British government to take "with the utmost seriousness any decision to send our servicemen to kill and be killed anywhere in the world".
The consent of Parliament should always be required for any substantial deployment of troops on active service, he said.
"Legitimacy means more than going through the right channels - it means doing the right thing," he said.
"We must not stoop to illiberalism - whether at Guantanamo Bay, or here at home with excessive periods of detention without trial," he said.
"We must not turn a blind eye to the excesses of our allies - abuses of human rights in some Arab countries, or disproportionate Israeli bombing in Lebanon.
"We are fighting for the principles of civilisation - let us not abandon those principles in the methods we deploy."