LONDON — Britons voted to quit the European Union after a divisive and bitter national referendum, according to predictions early Friday.
The Brexit debate, in which Britons weighed the strategic and trade benefits of bloc membership against costly European bureaucracy and the erosion of national sovereignty, was overshadowed by last week's murder of pro-EU lawmaker Jo Cox.
President Barack Obama had warned that the outcome of Thursday's referendum was "of deep interest to the United States" as well as a possible turning point for Europe.
The predicted result spooked financial markets, where stocks fell sharply.
Here's what you need to know about Brexit.
What is at stake?
In short, the future of Europe and the global order. Britain is the EU's second-largest economy, has a powerful military and exerts outsize influence in global affairs; it would be difficult to interpret Brexit as anything other than abandonment of the EU by one of its most important members.
Obama urged British voters to remain in the EU because its economic growth and counter-terrorism efforts will be "far more effective" with the U.K. as a member. But that intervention earned him a rebuke from some senior figures, including the mayor of London who said the president's position was "perverse."
Some predict a domino effect as other EU members consider their own departures, fearing an EU without Britain would be economically dominated by Germany. Countries such as Spain could also be forced to make hundreds of millions of extra dollars in annual contributions as budgets are realigned. A structural crisis is the last thing Europe needs amid the biggest refugee crisis since World War II, intractable economic woes and a nebulous security threat from international terrorism.
Only two days before the vote, British Prime Minister David Cameron said his country faced "an irreversible decision" on Europe. "There is no going back" from Brexit, he warned.
Cameron announced Friday that he would step down by the fall.
Why does it matter for the United States?
The United States advocates a strong, stable Europe. Even a smooth Brexit would see Washington building a new relationship with both Britain and a Britain-free remainder of the EU. However, some observers fear the rump of the EU could begin to unravel, weakening America's allies and bringing instability.
Despite a weakened "special relationship" between Washington and London, Britain remains America's primary transatlantic military ally and its role in Europe is strategically important to the U.S.
Why is this happening now?
Cameron was fulfilling an election promise to negotiate a better deal for his country in the European Union and to put the new terms to a national vote. It was an act of political expediency aimed at uniting two factions within of his Conservative party, which for decades has been bitterly divided over the issue of Europe.
An uptick in immigration and a long period of austerity have eroded British enthusiasm for the EU.
The trading bloc is arguably the modern world's first attempt at creating a "superstate" — a collection of nations that have agreed to give up meaningful aspects of their sovereignty in exchange for greater collective security and economic development.
A series of treaties have given courts and politicians in Brussels the power to change national laws and bring them in line with European standards. Up to 55 percent of Britain's laws and regulations are now set by Europe.
This forfeiture of national authority lies at the root of concern among many British voters.
How could it affect the U.S. election?
A perceived dissolution of the European Union would reinforce the isolationist views of many conservative Americans on the right. Politicians like Donald Trump would likely pounce on a Brexit as a reinforcement of the worldview he has championed on the campaign trail — that Europe is weak and its institutions are unable to cope with the realities of the modern age, and that border controls, protectionist trade policies and a reassertion of national sovereignty are essential to deal with the chaos of the outside world.
Meanwhile, internationalists — who in American politics are generally on the left, although there are pockets on the right as well — could only view this as a discouraging development.
Isn't Britain already separate from Europe?
Britain has long had a love-hate relationship with the EU. While it has supported the European bloc at key moments of its development, it has also kept the project at arm's length in major areas and has negotiated the most opt-outs of any member state.
In particular, it is a member of neither the euro currency zone — it retains the pound sterling — nor the border-free Schengen zone that allows travel without a passport.
Who are the key players?
Cameron is fulfilling his campaign promise — but has forcefully advocated in favor of Britain staying in the EU, along with left-leaning Labour Party leader Jeremy Corbyn.
They've been batting up against the "Leave" campaign championed by the likes of Boris Johnson, former mayor of London and Cameron's one-time friend at Oxford University.
Perhaps the most public face of the "Leave" campaign is Nigel Farage — the folksy leader of the UK Independence Party which has been a refuge for immigration-wary Britons.
What happens next?
A Brexit would be uncharted territory as no nation state has ever left the EU — although Greenland, an overseas territory of Denmark, quit in 1982.
A contentious issue in the campaign has been trade deals. "Out" campaigners say Britain could easily trade with the likes of the U.S. on its own terms. However, Obama specifically warned that Britain would "go to the back of queue" behind EU member states in negotiating new trade deals if it voted to quit.
A leading concern, especially among blue-collar voters, has been immigration — not only from the recent influx of refugees but also from Europeans taking advantage of rules allowing free movement within member states.
The economic impact of Brexit has also been hotly debated, with Britain's finance secretary threatening an emergency budget of tax increases and spending cuts if voters choose to quit the EU.
The history of the EU
There have been numerous attempts to create workable institutions that superseded national interests. The first serious attempt came in the wake of World War I. That bloody conflict reinforced the view among internationalists of the day that a binding pact among the nations of the world was necessary to maintain peace and stability. This resulted in the League of Nations, which died an unhappy death amid the rise of militant nationalist regimes in Europe and Asia in the 1930s.
The next serious attempt came at the end of the most disastrous conflict in human history, the Second World War. The United Nations was founded in 1945. Despite serious flaws in its conception — it was largely designed as a vehicle for maintaining the postwar geopolitical order — it continues chugging along.
The U.N. has evolved into a sprawling bureaucracy with a lot of "soft influence," but no hard power. Lacking in mechanisms for enforcement should any of its five permanent security council members disagree with the others, the U.N. has proven unable to carry out its primary function: stopping conflicts.
Enter the most recent attempt to shift power away from the nation state, the European Union. The EU has its roots in the Cold War, when Europe was dominated by the nuclear superpowers of the late 20th century, the United States and the Soviet Union.
With the horrors of the Second World War still fresh in their minds, a collection of European intellectuals and politicians fought for the creation of an organization that transcended the ethno-nationalist rivalries, which had led to so much bloodshed.