Media playback is unsupported on your device Media caption Theresa May: "We must renew the special relationship for this new age"

Theresa May's Philadelphia speech is hugely significant - arguably the biggest by a British prime minister in the US since Tony Blair's in Chicago.

Eighteen years ago - in 1999 - Mr Blair first advocated active military interventionism to overturn dictators and protect civilians.

Now, Mrs May has repudiated much of what he said then.

She talked of "the failed policies of the past", before making her crucial declaration of new foreign policy doctrine: "The days of Britain and America intervening in sovereign countries in an attempt to remake the world in our own image are over."

Of course, by saying that she was also overturning the approach of her predecessor, David Cameron. The current prime minister has also dismissed her predecessor's armed intervention in Libya.

Its aftermath - a failed state, far from recovery - haunts Britain still.

'Catastrophic' on Syria

This declaration of an apparently radical shift in policy by the prime minister should be read in conjunction with what appears to be an extraordinary British U-turn over Syria, which was set out in colourful terms by her foreign secretary only a few hours earlier.

Boris Johnson conceded the most bitter and recent failure of British foreign policy when he openly acknowledged what amounts to a fundamental defeat over Syria.

He called Britain's stance "catastrophic", shifting from the pledge of support over many years to the non-jihadist opponents of President Assad, to a position where Britain - together with the United States - retreated from the field and left it open to Russian military dominance.

Mr Johnson told a committee in the House of Lords that President Assad should now be permitted to run for election as part of a "democratic resolution" of the civil war - although he did also make clear there could be no sustainable peace in Syria as long as he remains.

He admitted the downsides of doing "such a complete flip-flop", but said the UK had been unable at any stage to fulfil its mantra that the Syrian president should go.

Image copyright Getty Images Image caption Boris Johnson said the UK's stance on Syria had been "catastrophic"

Mr Johnson was accepting Russia's victory - and at the same time swallowing the bitter pill of defeat for London and for Washington.

He said that had flowed from the refusal of the House of Commons, in August 2013, to back punitive British military action against President Assad for his use of chemical weapons - something the Syrian leader still denies.

Within days, President Obama had followed Britain in retreat.

Public appetite in both countries for almost any military intervention overseas had drained away after the years of intervention in Afghanistan, in Iraq, and in Libya.

It is very difficult to see circumstances in which Britain or the US will send forces against a sovereign government in the future.

Extremists - non-state actors - are almost the only acceptable target now.

New and important

The Foreign Office does not believe their political master was as explicit as I suggest, and believe that the essentials of British policy on Syria have not fundamentally changed.

Certainly, the prime minister did leave herself some wriggle room.

She argued against the sort of increased isolationism which President Donald Trump has championed, and urged the maintenance of the "special relationship" as a way to provide joint leadership in the world.

She said the two nations should not "stand idly by when the threat is real".

Nevertheless, the political presentation of British foreign policy by the prime minister and foreign secretary has deployed a distinctly new and sometimes startling language.

The direction being set in response to past failures and disappointments is different.

It may be largely a public recognition of some brutal realities, which have been emerging over several years, but it is new and important.