A Union Jack flag, left, and a European Union flag fly above Europe House in London on Tuesday. (Odd Andersen/Agence France-Presse via Getty Images)

Danielle Allen is a political theorist at Harvard University and a contributing columnist for The Post.

Nigel Farage, leader of the U.K. Independence Party, celebrated his victory in Thursday’s Brexit vote by declaring June 23 to be British “Independence Day,” which, he said, should become a national holiday. Post columnist George F. Will echoed that triumphal proclamation. Brexit promises to reinvigorate self-government, he wrote, and so “June 23, 2016, is now among the most important dates in postwar European history.”

But whatever you may think about Brexit, what it really shows is that a certain kind of political wisdom has gone out of the world.

For the past 13 years, since the U.S. invasion of Iraq, if not longer, people all over the world have pursued the business of regime change by thinking that the first step is to oust the old regime. Only once you’ve done that do you bother to think about what comes next.

You topple the desert dictators Saddam Hussein and Moammar Gaddafi; afterward, as if by afterthought, you develop a reconstruction plan and convert your military strategy to counterinsurgency techniques that require building functional societies. You overthrow Hosni Mubarak and his military government in Egypt, then focus on what the Muslim Brotherhood will do with power. Writing about the Arab Spring in the New York Times in 2013, political scientist Sheri Berman summed up the conventional view: “Establishing a stable democracy is a two-stage process. First you get rid of the old regime; then you build a durable democratic replacement.”

Here we are again with Brexit. The Brussels Eurocrats have, so it seems, been tossed out on their ears. Now what? What exactly does it mean? What sort of trade relations will Britain have with Europe? Can Britain even hold as a United Kingdom, or will Scotland, and possibly even Northern Ireland and Wales, peel off? Anxiety levels are skyrocketing.

The crazy thing about the invocations of Independence Day, and the rough and ready implicit references to the American Revolution, is that the basic lesson of 1776 has been missed.

Figuring out what’s coming next should precede declaring independence. Or put it this way: Constitutions come before revolutions.

Even Americans have lost sight of the fact that the American colonists understood they needed to know where they were going before they were ready to declare independence. They began the constitution-writing process in November 1775, colony by colony, thanks to a committee on which the architects of independence, John Adams of Massachusetts and Richard Henry Lee of Virginia, both served. This committee responded to a query from New Hampshire about the parlous condition of its local affairs by urging it to develop a constitution to replace the failing British administration.

That same month, Adams wrote a letter to Lee, which by the spring of 1776 was circulating throughout the colonies in both poster and pamphlet forms, in which he laid out the need to focus on the three separate branches of government — legislative, executive and judicial — and to balance them appropriately. New Hampshire followed his advice and passed its Constitution, the first of the colonies to do so, in January 1776. Adams’s own Massachusetts, where he was the chief justice of the Massachusetts Superior Court, also followed his advice and by that January had built new forms of governance intended to supplant British institutions.

In a proclamation for the Massachusetts General Assembly, a veritable first draft of the Declaration of Independence, written by Adams in January 1776, Adams lays out the basic principles of government:

“As the Happiness of the People alone, is the sole End of Government, So the Consent of the People is the only Foundation of it, in Reason, Morality, and the natural Fitness of things: and therefore every Act of Government, every Exercise of Sovereignty, against, or without, the Consent of the People, is Injustice, Usurpation, and Tyranny.”

In this text, his purpose is not yet to declare independence, but to help his compatriots understand what will be necessary to make the institutions they are designing functional. They have redesigned their executive and assembly, and even set up an army. The Massachusetts government has issued this proclamation about the basic principles of self-government so “That Piety and Virtue, which alone can Secure the Freedom of any People may be encouraged and Vice and Immorality suppress’d.” Preparing for independence requires not only taking the time to design the institutions that will operate post-independence, but also to prepare the manners and morals of the people for the eventual emergence of self-government.

On Adams’s February 1776 to-do list for the new session of Congress, the fourth item was “Government to be assumed in every colony.” “Declaration of Independency” was all the way down at No. 14.

Throughout the spring, Adams and Lee pursued the project of encouraging the colonies to design constitutions. The goal was to position the people to “pull down old tyrannies” and “erect new fabrics” all “at a single exertion.” This could be achieved only by taking the time to constitute before determining to revolt. On May 15, 1776, Adams secured passage in the Continental Congress of a resolution recommending that all the colonies adopt new forms of government. By the Fourth of July, not only New Hampshire and Massachusetts had new governing frameworks, but also South Carolina, Georgia, Virginia and New Jersey. Pennsylvania would launch its own constitution-writing process that same month.

The lesson from 1776 could not be clearer. Establishing a stable political regime is a two-stage process. First you figure out what shape you want your new regime to have; only then do you get rid of the old one. Wake up, world.