Something of that mood is in the air once again. Once again we live in an era of peace, prosperity and technological innovation. Once again, we take them all for granted. Once again, some are enthusiastic for a new adventure — and in Britain, Theresa May’s Brexit gridlock might make it possible. The prime minister has not managed to secure a parliamentary majority for the withdrawal plan that she negotiated with the European Union. Instead, she has postponed the vote, and this week the government started planning for a “no deal” exit. If this comes to pass, hundreds of trade and other agreements between Britain and the continent will simply become invalid on March 29, the day the negotiating clock runs out. Unless the E.U. decides to offer extensions, customs checks and tariff barriers will automatically snap into place — and nobody really knows what that means.

There could be shortages of food and medicine; already, the British state is buying extra refrigeration units. Flights could be grounded. Contracts could be invalidated. Millions of British citizens living in Europe, and millions of E.U. citizens living in Britain, will be in legal limbo. Britain’s relationships with its most important allies and economic partners could be permanently affected by a period of chaos, for many decades into the future.

But the real point is that nobody knows what will follow, because no nation in modern times has ever deliberately cut itself off from all of its most important trading partners, and ended all of its more important cultural and political relationships, overnight.