David Cameron has said the foreign policy of a Conservative government would be "proudly Atlanticist".
The Conservative leader was set to use a speech to a Washington think-tank to outline his approach to international affairs.
He also called for "decisive action" to prevent a new crisis emerging in the Balkans.
The Conservative leader was speaking at the Brookings Institution after visiting the Arlington National Cemetery.
"In Europe, we will never forget the sacrifices Americans have made for our liberty," he said.
"I and my colleagues represent a new generation of leadership in the Conservative Party.
"But the party I lead today in opposition, and which I hope to lead in government, is proudly Atlanticist, proud of the ties of history and family that bind our two nations."
Cameron said that America and the UK "must stand together against global terrorism fuelled by a perversion of the Islamic faith".
"I've seen our soldiers serving together in the deserts of Afghanistan and the dust of Iraq, and I pay tribute to their professionalism, their courage, and their comradeship," he added.
"The relationship between our two countries is indeed special.
"And it will remain special for any British government I lead - grounded in the long history we share together, and the ability to talk freely to each other as only old friends can.
"My view is clear: the cause of peace and progress is best served by an America that is engaged in the world.
"And the values we hold dear are best defended when Britain and the United States, and the United States and Europe, stand together."
Kosovo
Cameron also used his speech to warn that problems could arise if Russia blocks moves for Kosovan independence.
The EU and US support Kosovo, which has said it could declare independence from Serbia on December 10, but Moscow is backing Serbia in opposing any such move.
Cameron accused Russian president Vladimir Putin of taking a "less than helpful" approach.
"Let me make it clear: there could be a new crisis in the Balkans by Christmas," he said.
"That is a direct threat to our national security, and we must therefore take decisive action now to prevent it.
"We need to reinforce the military presence in the region now, by drawing on some of NATO's dedicated operational reserve, to prevent trouble later.
"Instability in the Balkans, with all the dangers that would bring, would be a threat to us all."
The Tory leader warned that a "lawless space" in the region "would be ideal ground for al Qaeda and others to take root".
"So preserving and enhancing stability in the Balkans is not just a moral imperative," he added. "It is fundamental to our national security."
Action
He called for "diplomatic action" to show President Putin "that we expect Russia to play a responsible role".
And he said that politicians in the Balkans "have a choice to make".
"You can choose a stable and peaceful future, with the full backing of the international community," he explained. "Or you can choose belligerence and conflict, with the full condemnation and united action of the international community."
Cameron also called for Washington's engagement in the region, saying the country had done much to promote stability in the Balkans.
"No country has done more in the last decade to bring peace and stability to that region than the United States," he added.
"No country has the clout - from Sarajevo to Skopje, from Belgrade to Tirana - that you do.
"As we cope with a new period of turbulence in the Balkans, you are entitled to look to the Europeans to carry their fair share of the burden, especially on the military front.
"But we enter this final, crucial chapter, your continued engagement will be crucial in seeing this endeavour through to success and ensuring that the gains of recent years are made permanent."
