Britain's Prime Minister Theresa May arrives to speak during the 2017 "Congress of Tomorrow" Joint Republican Issues Conference in Philadelphia, Pennsylvania, Jan. 26, 2017.
Britain's Prime Minister Theresa May arrives to speak during the 2017 "Congress of Tomorrow" Joint Republican Issues Conference in Philadelphia, Pennsylvania, Jan. 26, 2017.

British Prime Minister Theresa May says the days of the U.S. and Britain intervening in other nations to remake them in their image are over.

The prime minster spoke to a gathering of U.S. Republican leaders Thursday in Philadelphia, before meeting there with President Donald Trump.

May said it is in British and American interests to defend their values, but not go back to what she called the "failed policies of the past."

"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."

WATCH: Britain's May Discusses US, UK ‘Special Relationship’

WATCH: Britain's May Speaks as a ‘Fellow Conservative’

May also called for reform in such multinational institutions as the U.N. and NATO "to make them more relevant and purposeful." She said their members have to stop leaning on the United States.

"Sovereign countries cannot outsource their security and prosperity to America. And they should not undermine the alliances that keep us strong by failing to step up and play their part."

May said a Trump presidency can make the U.S. "stronger, greater and more confident," which she said is good for the rest of the world. She said British and American conservatives share the same principles.

Trump was greeted by a number of protesters when he arrived in Philadelphia. He later told fellow Republicans it was "nice to win" the state of Pennsylvania in the November election, which has traditionally gone to the Democrats.

Trump Protests Philadelphia
Thousands of demonstrators march in protest of President Donald Trump's visit in Philadelphia, Jan. 26, 2017.

He went on to reference a number of his most prominent policy aspirations, including his plan to build a wall along the Mexican border, and investigating alleged voter fraud in the 2016 election.

"We are going to protect the integrity of the ballot box and we are going to defend the votes of the American citizen. So important," he said.

The congressional Republicans are meeting for a three-day retreat, at which they will plan their legislative agenda for the coming years when they will control both houses of Congress and the White House.

White House visit

May is scheduled to visit the White House on Friday. She will meet with Trump and the two will give a joint news conference, the White House said.

Britain's Prime Minister Theresa May speaks during
Britain's Prime Minister Theresa May speaks during the 2017 "Congress of Tomorrow" Joint Republican Issues Conference in Philadelphia, Pennsylvania, Jan. 26, 2017.

She will be the first foreign leader to meet with Trump in Washington since he took office.

Both leaders have taken steps to reform their international relations, particularly through trade. Britain's pending exit from the European Union and Trump's withdrawal from the 12-nation Trans-Pacific Partnership will necessitate negotiating new trade agreements throughout the world.

May's plan for the E.U. exit includes placing a priority on controlling immigration, although she has not yet announced any policy details.

White House spokesman Sean Spicer said Trump hasn't yet decided whether he will cut funding to international organizations, like the United Nations, after media reports suggested the president was looking to reduce the role of the U.S. within those organizations.

Harsh words for Manning, Obama

Earlier Thursday, Trump called federal whistleblower Chelsea Manning an "ungrateful traitor," and slammed former President Barack Obama for releasing her from jail.

Bradley Manning ya no tiene nombre masculino y aho
FILE - Army Pfc. Bradley Manning is escorted to a security vehicle outside a courthouse in Fort Meade, Maryland, after a hearing in his court martial, Aug. 20, 2013.

Trump's comments apparently referred to an op-ed written by Manning and published in the British newspaper The Guardian, in which Manning says Obama compromised too much with his political foes and left behind "very few permanent accomplishments."

"This vulnerable legacy should remind us that what we really need is a strong and unapologetic progressive to lead us," Manning wrote. "What we need as well is a relentless grassroots movement to hold that leadership accountable."

Manning, who is transgender, was known as Bradley Manning before transitioning to life as a woman. She was found guilty in 2013 by a military court of leaking sensitive military documents to WikiLeaks, and subsequently sentenced to serve 35 years in prison.

During his final week in office, Obama commuted Manning's sentence. She will be released in May. At his final news conference as president, Obama said he felt "very comfortable" that Manning had been appropriately punished for her actions.