[This article has been updated to reflect the results of the vote in Parliament on Jan. 15.]

LONDON — After delays, stumbles and negotiations, Prime Minister Theresa May finally presented her plan for Britain’s withdrawal from the European Union to Parliament. Then, on Tuesday, British lawmakers resoundingly rejected it, 432 to 202.

It was one of the biggest defeats in the House of Commons for a prime minister in recent British history, and the humiliating margin has put Mrs. May’s government on the verge of collapse

Supporters of Brexit, as the withdrawal is known, had once promised that leaving the European bloc would be quick and simple. It has turned out to be neither. To understand why, it helps to understand the origins of the plan, and how that history is playing out today.