The Prime Minister said “new enemies of the West and our values have emerged” as she called on the US to renew its special relationship with the UK.
Speaking to Republican congressmen in Philadelphia, Theresa May said, by working together, Britain and the US can stop the “eclipse of the West” by China, India and emerging economies.
The Prime Minister also signalled support for some of Donald Trump’s key foreign policy priorities as she condemned “Iran's malign influence in the Middle East.”
The days of Britain and America intervening in sovereign countries in an attempt to remake the world in our own image are over
Mrs May echoed the President’s criticism of the Iraq invasion under Tony Blair and George Bush as she made a break from US-UK foreign policy of recent years.
She said: "It is in our interests – those of Britain and America together – to stand strong together to defend our values, our interests and the very ideas in which we believe.
"This cannot mean a return to the failed policies of the past. The days of Britain and America intervening in sovereign countries in an attempt to remake the world in our own image are over.
Theresa May urged Donald Trump to renew the UK-US special relationship
“But nor can we afford to stand idly by when the threat is real and when it is in our own interests to intervene.
“We must be strong, smart and hard-headed. And we must demonstrate the resolve necessary to stand up for our interests.”
Ahead of her meeting with the President today, Mrs May warned progress with Russia would help “to reduce Iran's malign influence in the Middle East”.
She said: "This is a priority for the UK too as we support our allies in the Gulf States to push back against Iran's aggressive efforts to build an arc of influence from Tehran through to the Mediterranean.”
Theresa May echoed the President’s criticism of the Iraq invasion under Tony Blair and George Bush
The Prime Minister’s speech was given five standing ovations
Stepping up to the President, the Prime Minister described the Iran nuclear deal – which Mr Trump has threatened to tear up – as "vitally important for regional security".
She also spoke out in support of the importance of international institutions like the UN, IMF and Nato, often the target of Mr Trump's scorn, in maintaining global peace and prosperity.
Mrs May urged the US to work with the UK to defend the West against the threat of “new enemies”.
She said: "New enemies of the West and our values – in particular in the form of Radical Islamists – have emerged.
The Prime Minister will become the first foreign leader to visit Mr Trump at the White House
Donald Trump will meet Theresa May later today
"And countries with little tradition of democracy, liberty and human rights – notably China and Russia – have grown more assertive in world affairs.
"The rise of the Asian economies – China yes, but democratic allies like India too – is hugely welcome. Billions are being lifted out of poverty and new markets for our industries are opening up.
"But these events – coming as they have at the same time as the financial crisis and its fall out, as well as a loss of confidence in the West following 9/11, and difficult military interventions in Iraq and Afghanistan – have led many to fear that, in this century, we will experience the eclipse of the West.
"But there is nothing inevitable about that. Other countries may grow stronger. Big, populous countries may grow richer. And as they do so, they may start to embrace more fully our values of democracy and liberty.
May meets Trump: Historic first meeting in pictures
Fri, January 27, 2017
The two leaders will spend about an hour in face-to-face talks in the Oval Office, where President Trump has restored a bust of Winston Churchill removed by predecessor Barack Obama.
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Prime Minister Theresa May speaks during the 2017
"But even if they do not, our interests will remain. Our values will endure. And the need to defend them and project them will be as important as ever.
"So we – our two countries together – have a responsibility to lead. Because when others step up as we step back, it is bad for America, for Britain and the world.”
The Prime Minister’s speech was given five standing ovations as she evoked the memory of Margaret Thatcher and Ronald Reagan’s historic relationship.
Asked by reporters whether she would struggle, as a reserved vicar's daughter, to strike a rapport with the brash property tycoon and reality TV star, Mrs May said: "Haven't you heard? Sometimes opposites attract."
Today Mrs May will become the first foreign leader to visit Mr Trump at the White House and the pair are expected to spend about an hour in face-to-face talks in the Oval Office, where the President has restored a bust of Winston Churchill removed by predecessor Barack Obama.