For the United States as global hegemon, maintaining a strong relationship with the United Kingdom has long been an important part of a grander picture. Between 1945 and 1989, the United States was locked in an existential confrontation with another global superpower, the Soviet Union. Arguably the Cold War's most important battleground, Europe dominated the United States' attention. Germany was split down the middle by the war's front line, and France had its own priorities. Southern Europe, though strategic, was often controlled by unsavory governments and lacked the necessary degree of military and political power besides. Instead, the United Kingdom, the launch pad for World War II's D-Day invasion, became the United States' primary partner. The countries' "special relationship," as Winston Churchill described it, required little effort to maintain: It was a natural friendship. The close relationship between British Prime Minister Margaret Thatcher and U.S. President Ronald Reagan helped them manage the Soviet Union's collapse in 1989.

After the collapse, the world changed, but the United Kingdom stayed useful. Now the sole superpower, the United States policed the world, stepping into regional confrontations where it saw fit. In its new role, the United States risked looking like a bully and therefore needed moral support to complement its overwhelming military power. The United Kingdom was happy to oblige in exchange for global influence, providing substantial troops from its world-class military to aid in various engagements. This has always been a bilateral relationship, thriving independently of the United Kingdom's membership in the European Union. Even so, as the Continental bloc became a more unified and powerful economic force, the United States enjoyed having a friend within it.

But Obama's presidency has coincided with another change in the global configuration. The rapid growth of various countries, led by China, in the past decade, combined with the Western world's economic travails since the 2008 financial crisis, has muddied the waters somewhat. From the United Kingdom's perspective, the greater array of economic opportunities worldwide makes joining the United States in every venture less of a foregone conclusion than it once was. In 2013, for example, the United Kingdom failed to support a U.S. intervention in Syria. A decade earlier, it would have been hard to imagine Blair doing the same thing to Bush.

Meanwhile, the United States has been trying to redirect its focus from the Middle East to East Asia, where it sees China emerging as its next rival. Here, the United Kingdom is a less obvious ally for two reasons, the first of which is geography. Unlike the policing projects of the past two decades, any potential confrontation between the United States and China will look more like the Cold War, requiring strategic rather than political support. When the Soviet Union was the adversary and Germany the battleground, the United Kingdom's location was ideal. In East Asia, on the other hand, Japan fills the role of the United States' permanent aircraft carrier, while Australia may prove a reliable Anglophone ally in the region. Second, the United Kingdom appears more conflicted with regard to East Asia than it has in previous rivalries. Courting Chinese investment last year, the United Kingdom directly contravened U.S. wishes and signed up for the Asian Infrastructure Investment Bank. To some extent, changing global currents seem to be pushing the old friends apart.

Today, some of the tenets underpinning the countries' alliance are no longer in place. Even the United Kingdom's role in countering Russia's recent activities in Ukraine has been limited so far, since it is not among the countries conducting negotiations. Although its role could very well increase in the future, the United Kingdom has, to date, been a bit player. Instead, it is the United Kingdom's (previously secondary) role as an influential force in Europe that the United States now values most in its ally. Because the United States' global ambitions are best facilitated by unity among the Western powers, a divisive British exit from the European Union could be a serious hindrance for Washington. As the United States tries to focus its attention on Asia, a splintering European continent is the last thing it needs. For as long as the European Union holds together, the United States will want as many friends on the inside as possible to sway the bloc's direction in a way that will promote America's goals. To this end, when Obama speaks to the British public in the coming days, he will advise them to choose "remain." And when the queen sits down to birthday lunch with her 12th U.S. president, she may find fewer areas of agreement than she had with his 11 predecessors.