LONDON—Red double-decker buses still cruise up and down the Strand, the guards stand up straight in front of Buckingham Palace, and the queen rides her horse-drawn carriage to the opening session of Parliament every year. But beneath this seemingly immutable surface, Britain is changing with surprising speed.

For one, the discontent with “establishment” politics that has convulsed so many European countries has finally reached the British Isles. There have been no mass marches here, as in Spain, and no anarchist riots, as in Greece. Instead, large numbers of Britons are cheerfully telling pollsters that they aren’t planning to vote for the Conservative Party in May’s general election, and that they don’t like the Labour Party or the Liberal Democrats either. If they are Scottish, they might be planning to vote for the Scottish National Party, which hopes to immediately reinvigorate its campaign for Scottish independence. If they are English, they might vote for the anti-European, anti-immigration United Kingdom Independence Party, or maybe for the Greens.

Nothing wrong with that in principle, but it’s hard to imagine how a four- or five-way split could produce a stable government, especially in a country that has been ruled by Labour or the Tories for most of the past century. Priorities could change dramatically. At a recent London breakfast meeting, a room full of experts spun out every possible scenario—Tory-UKIP coalition? Lab-Lib-ScotNat coalition? Minority government, with new elections soon after?—and tried to work out what each would mean. The conclusion: Britain might take a radical step to the left, it might take a radical step to the right, it might leave the European Union, it might break up altogether. In other words, no one could predict anything, except that Britain would be occupied by its own internal arguments for a very long time.

This second development is not unrelated to the first: Suddenly, without much discussion, it seems as if Britain—a nuclear and conventional military power, a staunch U.S. ally, a pillar of NATO—has lost its historic interest in foreign policy. The drift began in 2009, when David Cameron withdrew his Conservative Party from the Christian Democratic mainstream in the European Parliament, a decision that instantly gave him less access to the most important European leader, Angela Merkel. Soon after, he announced his intention to hold a referendum on Britain’s EU membership, immediately affording him less credibility in Europe, too: If the British were already halfway out the door, why bother talking to them at all?

But Britain’s gradual fade from the world stage was abruptly accelerated in 2013, when Cameron declared he would support U.S. airstrikes in Syria, called a parliamentary vote for support—and lost. He dropped that idea, apparently spooking President Obama, who dropped it, too. Both were forced to reconsider after ISIS captured large chunks of Syrian and Iraqi territory. But even now the British contribution to the anti-ISIS campaign consists of eight planes, hardly an overwhelming commitment. Defense spending is likely to be frozen next year. And when Merkel flew to Minsk to meet Russian President Vladimir Putin last month, she took the French president along as a fig leaf, leaving the British behind.

Political weakness is part of the problem: The Conservatives are by no means guaranteed re-election, and the foreign secretary is of the opinion, he reportedly recently said, that there are “no votes in defence.” The Iraq war turned many in Britain off the idea of ever cooperating with Americans, and the European recession explains a lot of the doubts about Europe. Provincialism is a factor, too. In a country where the political class is consumed by an argument over the constitutional status of Scotland, events in the Middle East or Ukraine seem far away.

Which is all very well, until it turns out that they aren’t far away at all. Russian military jets have lately taken to buzzing British airspace, even crossing into the paths of passenger planes. Britain’s enormous financial exposure to Russia may already be a grave security risk. Middle Eastern terrorism has a way of looping back to London as well as Paris. More to the point, both ISIS and Putin want to destroy the global economic and political system in which Britain has long thrived. Maybe there aren’t any votes in defense, but do the British really believe they will be better off in a world where they have no influence?

It’s hard to say, since none of them is really talking about it. Once upon a time, Britain was said to have acquired its empire in a “fit of absent-mindedness.” It may be about to lose its place on the world stage in exactly the same way.