British Prime Minister David Cameron didn’t want to hold a vote on Brexit at all. But in 2014, he faced growing pressure from the populist right over immigration and Britain’s EU membership. To mollify dissenters in his own party and stop the rise of the far-right UK Independence Party, Cameron promised to hold a referendum on leaving the EU if his Conservative Party won the 2015 election.
The Conservatives surprised pollsters by winning an outright majority in Parliament, and Cameron kept his promise. But he wasn’t personally in favor of exiting the EU, and he campaigned vigorously for a "Remain" vote. At the same time, he allowed other members of his government to campaign on the other side. This created the spectacle of senior members of the UK government, from the same party, campaigning on opposite sides of one of the biggest issues in British politics in decades.
Not long after Thursday's results became clear, Cameron announced that he would resign by October. It's unclear what will happen next, but one likely outcome will be for Cameron to be replaced by Boris Johnson, a member of Cameron's Conservative Party and former London mayor who campaigned for a Leave vote. It's also possible that the Conservative Party will splinter so badly that no one is able to claim a parliamentary majority, forcing early elections.