Tony Blair has failed to influence the policies of George Bush's White House in any significant way, despite his unwavering support for the US president, a leading foreign affairs think tank said today.
Delivering its verdict on ten years of foreign policy under Mr Blair, a Chatham House briefing paper said his legacy would be defined by the "terrible mistake" of the war with Iraq.
It said Mr Blair was now paying the price for setting too much store by his relationship with Mr Bush and warned that his successor would have to strike a new foreign policy balance between Europe and the US.
"The post-9/11 decision to invade Iraq was a terrible mistake and the current debacle will have policy repercussions for many years to come," the paper said.
"The root failure of Tony Blair's foreign policy has been its inability to influence the Bush administration in any significant way despite the sacrifice - military, political and financial - that the United Kingdom has made.
"Tony Blair has learnt the hard way that loyalty in international politics counts for very little."
The paper, written by outgoing Chatham House director Victor Bulmer-Thomas, said Mr Blair's first term as Prime Minister had been a "qualified success" - demonstrating Britain's European credentials while forging a close working relationship with US president Bill Clinton.
However his response to the 9/11 attacks in 2001 and decision to back Mr Bush over the invasion of Iraq marked a "watershed" in British foreign policy.
"This was without a shadow of doubt the defining moment of Blair's foreign policy - indeed, the defining moment of his whole premiership. It will shape his legacy - for better or for worse - for many years to come," the paper said.
In the absence of support in the United Nations for a humanitarian intervention in Iraq, the paper said it had been a "terrible mistake" to rely on Saddam Hussain's weapons of mass destruction (WMD) as a justification for war.
It said "the jury is still out" on the extent to which Mr Blair knew that the claims about Iraqi WMD were "overblown or even fabricated".
However, by the time of the invasion, it was clear Britain was not sure of its case, the diplomatic options had not been exhausted, and the UK national interest was truly engaged.
"Under these circumstances, the only thing that could have rescued Tony Blair was a swift and successful establishment of a democratic government in Iraq, and that did not happen," the paper said.
It dismissed Mr Blair's claims that unwavering support for the US in public would bring influence in private, accusing the Prime Minister of over-estimating his political capital in Washington.
"The bilateral relationship with the United States may be 'special' to Britain, but the US has never described it as more than 'close'," it said.
"The best that could be hoped for is that Britain would ally itself with one of the Washington factions that ultimately prevailed in the internecine struggle for presidential support."
The paper said Mr Blair's successor would not be able to make the same mistake, and would instead have to develop a closer relationship with Europe.
"For the foreseeable future, whoever is Prime Minister, there will no longer be unconditional support for US initiatives in foreign policy," it said.
Elsewhere it said it was "unforgivable" that Mr Blair had failed to foresee the consequences of a Taliban resurgence in Afghanistan, given the impact on heroin use on Britain's streets.
In contrast, it said his most positive legacy in foreign affairs would be his determined advocacy of the need to tackle climate change.
"While it is easy to criticise the gap between his rhetoric and Government actions at home in mitigating climate change, there is no doubt that Blair's powers of persuasion have been very effective in pushing this issue up the international agenda," it said.