Nearly 52% of British voters in the June 23 referendum on European Union (EU) membership answered that the United Kingdom (UK) should leave the EU. Among a complex pattern of supporters and opponents of Brexit, the vote pitted Prime Minister David Cameron, who led the campaign to remain, against many me mbers of his own Conservative Party, including in his Cabinet. Prime Minister Cameron subsequently resigned and Theresa May took over as the UK's new prime minister on July 13, 2016. The vote was the culmination of a decades-long debate in the UK about the country's EU membership. The UK joined the precursor to the modern EU in 1973, but has long been considered one of the most "euroskeptic" members, having "opted out" of several major elements of European integration, such as the euro currency and the passport-free Schengen Zone. One of the central arguments made by the Leave campaign was that the EU had steadily eroded the UK's national sovereignty by shifting control over many areas of decisionmaking from Parliament to Brussels. Analysts also attribute the result to the prevalence of concerns about high levels of immigration to the UK, which the Leave campaign linked to the EU requirement for the "free movement of people" among member states. Leave campaigners further argued that EU bureaucracy and regulations held back the UK's economy, and that in the long-term the country would be better off economically outside the EU. Although technically the referendum is only advisory for Parliament, the government asserted at the start of the campaign that it "would have a democratic duty to give effect to the electorate's decision" and that it would do so quickly. There is no precedent for a country withdrawing from the EU so a high degree of uncertainty exists about how the separation might work. The vote does not force the UK out of the EU immediately. Under its treaty framework, a member country may withdraw from the EU by invoking Article 50 of the Treaty on European Union, opening a two-year period in which the two sides would attempt to negotiate a withdrawal agreement. There is no pre-set timeframe for the notification that begins this process. While some European officials have urged moving quickly in order to limit the uncertainty surrounding Brexit, leading advocates of the Leave campaign have conveyed their view that there is no hurry to initiate the withdrawal process. The main purpose of the withdrawal agreement would be to settle transition arrangements in policy areas, such as the single market, that are covered by the EU treaties. Until the negotiation is concluded, the UK remains a member of the EU and subject to its rules. Details about the future arrangement of the relationship between the UK and the EU are likely to be negotiated as a separate agreement. Many observers believe that the process of negotiating these agreements could take considerably longer than two years to complete. As expressed prior to the referendum by the UK government itself, "a vote to leave the EU would be the start, not the end, of a process. It could lead to up to a decade or more of uncertainty." In addition to uncertainties about the process and timeline for withdrawal, analysts have speculated on the possible broader consequences of Brexit. Many economists have expressed concerns that Brexit could cause an economic shock that leaves the UK facing weaker economic growth, higher inflation, job losses and depreciation of the pound (which has already happened), with potentially significant negative consequences for the U.S. and global economies. Advocates of Brexit have maintained that such economic fears are greatly exaggerated. Meanwhile, observers have noted growing unease among some of the many multinational corporations that have chosen the UK as their EU headquarters, who now face a period of uncertainty about UK's trade and economic arrangements and the corresponding legal and regulatory frameworks. The regional dimensions of the referendum result have also fueled questions about the future of the UK's political union. The Brexit vote has triggered a renewed push by Scottish leaders for Scotland (where 62% of voters supported remaining in the EU) to separate from the UK. It could also raise questions for Northern Ireland (where nearly 56% voted to remain) about its border arrangements with the Republic of Ireland. Many observers have concerns that Brexit could prompt a wider unraveling of the EU. At a time of growing skepticism toward the EU in many member countries, the UK's departure could lead to more calls for special membership conditions or referendums on membership in other countries. Many U.S. officials and Members of Congress view the UK as one of the United States' closest and most reliable allies. In terms of broad foreign policy issues, Brexit could have a substantial impact on U.S. interests. With the UK commonly regarded as the strongest U.S. partner in Europe and a partner that usually shares U.S. views, U.S. officials have conveyed concerns that a UK break from the EU would reduce U.S. influence in Europe, weaken the EU's position on free trade (because the UK generally acted as a leading voice for economic liberalism in EU debates about trade and the single market), and make the EU a less reliable partner on security and defense issues (because British participation is widely regarded as essential for efforts to develop more robust EU foreign and defense policies). Brexit need not cause a dramatic makeover in all aspects of the U.S.-UK relationship, however. NATO remains the preeminent transatlantic security institution, and the UK will remain a leading member of NATO. Despite recent U.S. concerns about the possible effects of cuts to UK defense spending and reductions in the size and capabilities of the British military, U.S. defense planners have long viewed the UK as one of the most capable European allies in terms of well-trained combat forces and the ability to deploy them. Observers also note that the United States and the UK have long tended to have similar outlooks on issues such as the use of force, the development of military capabilities, and the role of NATO. Close cooperation with the UK on NATO issues can be expected to continue, including with regard to the joint deployment of additional forces for the reassurance of Poland and the Baltic States (and the deterrence of Russia), and in operations to combat the Islamic State. A 1958 U.S.-UK Mutual Defense Agreement established unique cooperation with regard to nuclear weapons, and the United Kingdom and the United States are also key partners in terms of defense industry cooperation and defense sales. There is no obvious reason why Brexit would change these aspects of the relationship. Most analysts and officials also agree that U.S.-UK intelligence and counterterrorism cooperation is close, well established, and mutually beneficial. UK agencies routinely cooperate with their U.S. counterparts in sharing information, and U.S. and British law enforcement and intelligence agencies regularly serve as investigative partners. The bilateral intelligence and counterterrorism relationship is likely to continue as an important pillar of the relationship. At the same time, experts assert that Euro-Atlantic cooperation on a range of security concerns may not be immune to the effects of Brexit. Broadly speaking, some suggest that Brexit undermines the notion of "Western unity" in the face of threats such as terrorism and Russian aggression, weakening the collective ability of the United States and Europe to deal with such threats. Additionally, any wider fracturing of the EU might cause economic disruption and create upheaval in numerous other ways that could pose a serious challenge to U.S. foreign policy. Much of the uncertainty that exists in the aftermath of a vote to leave the UK relates in some way to the UK's trade and economic arrangements and the corresponding legal and regulatory frameworks. Over the past three years, the UK economy has consistently grown in the 2% to 3% range. According to the IMF, the UK economy is slowly regaining performance levels it experienced prior to the 2008-2010 global financial and economic crises. Trade is equivalent to about 60% of the UK economy, in large part due to reduced trade barriers with the EU through the EU's Single Market. The UK is also an important component of the EU economy. With the UK, the EU's GDP is greater than that of all other individual economies, including the United States and China. Without the UK, however, EU GDP would rank behind that of the United States and China. Trade with the EU accounted for 44.6% of the UK's total exports of goods and services and 53.2% of its imports of goods and services in 2014, as indicated in Figure 1 . According to the British Treasury, the Single Market has facilitated the growth and development of a complex web of economic and financial ties between the UK and the EU. These connections have assisted in the growth of the services sector in the UK, which accounts for about 80% of UK GDP, including service industries and complex manufacturing activities that rely on cross-border supply chains, largely with EU countries. In part reflecting these extensive ties, the stock market value of UK bank equities fell by more than 10% immediately following the vote; trading was briefly suspended on Monday, June 27, 2016. According to various analyses, Brexit could have an immediate and long-term effect on the UK and EU economies. In particular, financial markets reacted negatively to the referendum on the UK exit from the EU. Major stock indexes plunged in value in Asia, Europe, and on Wall Street. Within two weeks, however, most stock markets had recouped their initial losses, but uncertainty about long-term prospects for the British economy continues to feed through financial and economic linkages to developed and emerging economies. In addition, central bankers in Switzerland, Japan, and Europe reportedly raised concerns over the possibilities of financial market instability associated with the vote in favor of Brexit. Group of Seven finance ministers and central bankers, meeting in Switzerland for pre-scheduled sessions, issued a statement indicating, "We affirm our assessment that the U.K. economy and financial sector remain resilient and are confident that the U.K. authorities are well-positioned to address the consequences of the referendum outcome."