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LONDON — As British Prime Minister Theresa May visits President Donald Trump in the White House on Friday, it’s a safe bet there will be talk of the “special relationship” between Britain and the United States. And much of it will come from the British side.

May can take satisfaction from being the first foreign leader to visit Trump since his inauguration one week ago. Nevertheless, some experts question just how "special" the relationship between the two countries is.

Some historians say the phrase “special relationship” gains currency particularly when Britain is in need of help — as is the case now. It can be the diplomatic equivalent, of, “Hey old buddy, old pal, can you give a friend some help?”

In this case, May, having made it clear that Britain will leave the European Union’s free trade area, is in urgent need of a quick trade agreement with the U.S. to limit the economic damage, according to the author Roger Hermiston, who has written about Churchill’s World War II leadership.

May is going as “a supplicant, with a begging bowl,” he said.

But sometimes the relationship works the other way. When President George W. Bush took office in 2001, Tony Blair was the senior figure, having been prime minister since 1997. He was a source of strength when Bush, new to office, was shattered by the terrorist attacks of Sept. 11, 2001, said Anthony Seldon, who is writing a book on the U.S.-U.K. relationship, called “Special.”

The phrase was not coined until 1946, but Seldon dates the special relationship to 1919 and the U.S. entry into World War I. The idea is that the U.S. and Britain are bound by history, culture and language more closely to each other than they are bound to other countries.

Over the years, there have been vital partnerships between presidents and prime ministers — some for better, some for worse. Here is a look at three of the most notable ones.

The Royal Navy During The Second World War, President Franklin D. Roosevelt and Prime Minister Winston Churchill after Divine Service on board HMS Prince of Wales, in the North Atlantic off the coast of Newfoundland, Aug. 10, 1941. IWM via Getty Images

Franklin Roosevelt-Winston Churchill

This partnership, which did much to save the world from Nazism, began poorly. But in the end, it was Churchill, the British prime minister, who coined the phrase “special relationship.”

The two men met at a dinner party in 1918 and did not get along, said Hermiston, the author of “All Behind You, Winston: Churchill’s Great Coalition 1940-45.”

At the dinner party, Churchill mostly ignored Roosevelt, who took umbrage. And Roosevelt was further offended when, later on, Churchill wrote that the New Deal was a “ruthless war on private enterprise.”

Yet in the early years of the war it was Roosevelt who resumed contact, writing to Churchill on September 11, 1939 to say he was glad Churchill was back at the Admiralty.

Churchill — knowing how badly Britain needed America’s help — “set out to woo, to flatter, to be a supplicant to Roosevelt,” Hermiston said.

“This, I think, was Churchill’s greatest achievement,” he said.

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The prime minister took the task quite seriously. “No lover ever studied every whim of his mistress as I did those of President Roosevelt,” he said.

The two men eventually developed a close relationship. Roosevelt biographer John Gunther has estimated that, between 1939 and 1945, they exchanged 1,700 letters and cables, and met 11 times. Churchill said he and Roosevelt had about 120 days of close personal contact.

And once, at the White House, Roosevelt wheeled himself into the room where Churchill was staying and found the prime minister, in the words of Roosevelt’s aide Harry Hopkins, “stark naked and gleaming pink from his bath.”

Churchill’s precise words are a matter of debate, but apparently he didn’t miss a beat. “The prime minister of Great Britain has nothing to hide from the president of the United States,” he reportedly said.

President Ronald Reagan, right, and Britain's Prime Minister Margaret Thatcher, share a laugh during a break from a session at the Ottawa Summit at the Government House in Ottawa, Canada, July 21, 1981. AP

Ronald Reagan-Margaret Thatcher

Fellow Cold warriors and free marketeers Ronald Reagan and Margaret Thatcher were so in tune ideologically that, even when they first met, they finished each other’s sentences, said Nicholas Wapshott, who has written a biography of Thatcher.

“They enjoyed what might today be called an office marriage,” Wapshott said. “And they would refer to each other as if husband and wife.”

When Thatcher scolded Reagan about the 1983 U.S. invasion of Grenada, a former British territory, Reagan knew he was in trouble.

"If I were there, Margaret,” he said on the phone, “I’d throw my hat in the door before I came in.”

They remembered each other’s birthdays, and those of each others' spouses.

“They were intensely protective of each other, but particularly Thatcher of Reagan,” Wapshott said.

British Prime Minister Margaret Thatcher dances with President Ronald Reagan on Nov. 16, 1988. following a state dinner at the White House. M. Sprague / AFP - Getty Images, file

During Reagan’s second term he was damaged by the Iran-Contra scandal, in which administration officials facilitated the sale of arms to Iran in violation of an arms embargo, hoping to use the proceeds to fund right-wing Contra rebels in Nicaragua.

Reagan’s reason for knowing nothing about it was that he had not been paying attention.

“Thatcher flew to America and went the rounds of the morning chat shows, wagging her finger and telling her interviewers — and the American people — that they were jolly lucky to have a president as true and honest as Reagan,” Wapshott said.

In exchange, he said, Thatcher acquired significant world power by association, the author said.

When Reagan died in 2004, Wapshott said, Thatcher “draped herself across the casket at the ceremony in Washington Cathedral,” Wapshott said. “This was not play acting. She was devastated by his death, almost as much as she was by the death of her own husband, Denis.”

George W. Bush-Tony Blair

Tony Blair is the only prime minister to have forged a close relationship with two U.S. presidents — first Bill Clinton and then George W. Bush.

Clinton and Blair had a natural affinity, said Seldon, the author of the forthcoming book on the special relationship. Both were young and married to lawyers. More importantly, they both had pulled their parties from the left back to the center.

British Prime Minister Tony Blair listens as President George W. Bush speaks at a news conference in the Rose Garden of the White House May 17, 2007 in Washington. Win McNamee / Getty Images, file

For Blair and Bush, though, it was different, Seldon said.

They “didn’t have the same natural bonds of personality that attracted Blair and Clinton,” he said. “They were drawn together more by the force of what they faced.”

Both men viewed the threat from Islamic militants in terms of good versus evil, he said. Both were were religious, and saw themselves as upholding morality and integrity against the forces of darkness, he said.

“They became deeply close,” he said.

Seldon dismisses any suggestion the U.K. was the supplicant in this case.

On Sept. 20, 2001, when Blair visited New York and Washington, he was the stronger figure, Seldon said. He had intervened successfully in Sierra Leone and in Kosovo, and was confident in his leadership.

And so the two leaders participated together in the invasion of Iraq in 2003, a controversial decision widely seen as disastrous and one that scarred both their legacies.

“When it started going wrong, that brought them closer together,” Seldon said. “They were indelibly linked together by that decision in March 2003. They knew their destiny lay in working through that side-by-side.”

So how 'special' is the relationship?

But scholars debate just how real the special relationship is.

“I think perhaps the very notion of a ‘special relationship’ is a British myth,” said Wapshott. “I have never heard an American talk about it except as a patronizing flip to the Brits.”

But Seldon begs to differ. The relationship is close, he said, and it benefits both sides.

“I think the U.S. has done best on the world stage when it has worked with the United Kingdom,” he said.