In his historic "Iron Curtain" speech delivered in Missouri 70 years ago, Britain's former Prime Minister Winston Churchill predicted the beginning of the Cold War and popularized the term "iron curtain" for the division of Europe.

In the same speech, Churchill, whose mother was American, also coined the term "special relationship" to describe the unique ties between Britain and the US. While the relationship had been close before, World War II only deepened the ties between Washington and London.

"The British wanted to continue the very close cooperation and consultation and joint decision-making that had developed during World War II," said John Harper, a professor of American foreign policy at Johns Hopkins University's School of Advanced International Studies in Bologna. "It has always been more of a British idea than an American idea."

Wars in Afghanistan and Iraq

During the ensuing Cold War, ties between both countries remained exceptionally close, not just in military but also in political and economic terms. Even after the Cold War had ended, the special relationship reached a new peak under British Prime Minister Tony Blair and US Presidents Bill Clinton and George W. Bush, with London playing a key role in the US-led wars in Afghanistan and Iraq.

Winston Churchill advocated for the special relationship

One of the reasons that the nature of the relationship has changed considerably since then is partly a consequence of British public opinion, which has "turned against the link with the United States a bit because of the wars," said Anand Menon, a scholar on British foreign policy at King's College in London.

But it is not just British sentiment that has cooled toward the US.

Britain's increasing skepticism about the European project, which has led to a heated discussion about a so called 'Brexit' and will culminate in a referendum over the country's membership in the EU in June, has not gone unnoticed across the pond.

Brexit and China

"The Americans are not happy about that," said Harper. Washington has always supported Britain's membership in Europe and wants London to play a strong role in European affairs, if only out of self-interest, because traditionally Britain's positions on economic and political matters were more aligned with those of Washington than with many other European countries. As a result, "if Britain were to leave Europe that would definitely strain the relationship with the US."

What also ruffled feathers in Washington is Britain's strong effort to cultivate close financial ties with Beijing and woo Chinese investments. Last September, Britain's Chancellor of the Exchequer George Osborne urged his Chinese hosts in Shanghai to "create a golden decade for the UK-China relationship." One month later, British Prime Minister David Cameron rolled out the red carpet for Chinese President Xi Jinping in London.

"The message to the Americans is we are going to have our economic relationship with China and we won't necessarily support you with your problems with China regarding the territorial disputes in the South China Sea," said Harper. "That really goes against the spirit of the special relationship."

"David Cameron, partly because of domestic political pressure, is determined to show that the current British government is very keen to be very good friends with China," said Menon. "It is very keen to show that we are a global player. And there is less emphasis than in the past that we are America's best friend and that that relationship completely dominates our foreign policy."

Other partners

The Chinese president's visit in London raised eyebrows in the US

Washington for its part has also looked elsewhere for partners in Europe for some time now: to Germany and France.

"When it comes to foreign policy over Ukraine and economics, they talk to Merkel, and when it comes to military stuff, they will talk to the French," Menon noted.

The fact that both Britain and the US have downgraded the importance of the special relationship does not mean, however, that it is defunct. In the military and intelligence arena, the ties between both countries remain uniquely close, which is partly due to the fact that in these areas the cooperation is very much institutionalized.

But beyond those spheres, the US and Britain have grown apart since Churchill's remarks 70 years ago, Harper said. "It would be wrong to say the special relationship is dead, but at the moment it is not very vibrant."