LONDON — Britain’s historic vote to leave the European Union is already threatening to unravel a democratic bloc of nations that has coexisted peacefully together for decades. But it is also generating uncertainty about an even bigger issue: Is the post-1945 order imposed on the world by the United States and its allies unraveling, too?

Britain’s choice to retreat into what some critics of the vote suggest is a “Little England” status is just one among many loosely linked developments suggesting the potential for a reordering of power, economic relationships, borders and ideologies around the globe.

Slow economic growth has undercut confidence in traditional liberal economics, especially in the face of the dislocations caused by trade and surging immigration. Populism has sprouted throughout the West. Borders in the Middle East are being erased amid a rise in sectarianism. China is growing more assertive and Russia more adventurous. Refugees from poor and war-torn places are crossing land and sea in record numbers to get to the better lives shown to them by modern communications.

Accompanied by an upending of politics and middle-class assumptions in both the developed and the developing worlds, these forces are combining as never before to challenge the Western institutions and alliances that were established after World War II and that have largely held global sway ever since.