U.S. President Donald Trump heads to London for his second visit in less than a year. The trip is being touted by both sides as an opportunity to strengthen their long established “special relationship.”

That phrase has historically been used to describe the UK-U.S. alliance, even if it’s not always the political reality, as CGTN’s Gerald Tan reports.

The United States and Britain share one of the most enduring alliances in modern history. From politics to economics and culture, the two are inextricably linked.

Language is perhaps the most obvious commonality. This not only eases direct communication but also fosters a shared love for works in English—think Shakespeare, Hollywood, even Harry Potter.

In terms of global security, they’re both founding members of NATO and permanent members of the U.N. Security Council. And they remain committed to the UKUSA Agreement, which allows robust intelligence sharing.

On a business level, the U.S. and UK are each other’s largest investor. Companies on both sides of the Atlantic pour money into sectors from manufacturing and IT to financial services.

These close bonds are the basis of the so-called “special relationship.” The term was coined by former British Prime Minister Winston Churchill after World War II and has been invoked by successive leaders since.

The strength of Anglo-American ties often boils down to the personalities of who’s occupying the White House and 10 Downing Street. And history provides many examples.

During the Second World War, Winston Churchill called on Franklin Roosevelt for support, forging the modern military alliance that continues to this day.

At the height of the Cold War, John F. Kennedy ignored the objections of his advisors and sold Polaris nuclear missiles to the UK when Harold Macmillan insisted on an independent nuclear deterrent.

In the 1980s, Britain’s first female Prime Minister Margaret Thatcher found a close partner in Ronald Reagan. Their views on capitalism and free markets were so close that an aide dubbed them “political and philosophical soulmates.”

Tony Blair faced intense criticism for supporting George W. Bush decision to invade Iraq after 9/11. An inquiry after the war concluded Iraq did not pose an urgent threat to the UK.

Still, things aren’t always rosy. Although Barack Obama got along with David Cameron, the U.S.-UK relationship seemed taken for granted. Obama appeared more concerned with Germany, Iran Syria, and China, announcing a U.S. pivot to Asia.

So how special is that relationship now? Donald Trump has famously criticized Theresa May for her handling of Brexit and repeatedly accused UK intelligence services of spying on him. This is his second visit to Britain as president, just as May prepares to leave Downing Street. Her successor will play a crucial role in shaping the dynamics that define U.S.-British relations, for better or worse.