If Brexit happens, Britain will again have to find new ways to exert influence and secure its interests.

Yet Britain remained a permanent Security Council member and a major player in NATO. It found an expanded role through the EU, not least as a connecting point between the U.S. and Europe.

The issue of Britain’s international identity dates to the end of World War II. Former U.S. Secretary of State Dean Acheson once said the British had “lost an empire and not yet found a role.”

Prime Minister-to-be Boris Johnson once accused those in favor of remaining in the European Union of “woefully underestimating this country and what it can do.” Yet Iran’s seizure of a British-flagged ship has provided a reminder that politically and militarily, Britain is unable, for instance, to provide naval protection for its merchant shipping in the Gulf. It does retain influence, but this has much to do with its alliances.

As Brexit looms, Britain is about to embark on a fundamental redefinition of its place in the world – just as rising international tension over Iran and the Gulf underlines the challenges it will face in doing so.

For the headline writers, it might be called the Week of the Three B’s: Britain, Boris (as in Boris Johnson, its flamboyant new prime minister), and Brexit (the political issue that has brought him the office he has long sought).

But something of larger, long-term significance is also happening with Mr. Johnson’s entry into 10 Downing Street. Britain is about to embark on a fundamental redefinition of its place in the world. And the non-Brexit crisis at the top of his inbox – rising international tension over Iran and the Gulf – has underlined the challenges it’s likely to face in doing so.

It has provided a reminder that politically and militarily, Britain on its own is no longer one of the world’s major powers – unable, for instance, to provide naval protection for its own merchant shipping in the Gulf. It does retain considerable influence – in the British phrase, “punching above its weight.” But this has much to do with its international alliances.

In a barnstorming performance three years ago, Mr. Johnson closed the national referendum campaign on Brexit – taking Britain out of the European Union – by accusing those in favor of remaining of “woefully underestimating this country and what it can do.” He concluded: “If we vote leave, this could be our country’s Independence Day!” With echoes of then-candidate Donald Trump, his message was essentially: MGBGA, “Make Great Britain Great Again.”

Mr. Johnson clearly still believes he can make a success of Brexit. Indeed, he struck much the same tone after winning the Conservative Party election to succeed Prime Minister Theresa May.

The question of alignments

But the Iran crisis has highlighted the limits of Britain’s power in today’s world, and the difficult choices it will have to make in carving out a post-Brexit identity.

Chief among them is likely to be how Britain aligns itself with major EU countries like Germany and France on the one hand, and with the United States on the other, especially since the U.S. is viewed by Brexit supporters as a potentially expanded trade partner to compensate for the economic hit it is likely to take in leaving the EU. The Iran crisis has brought that choice into sharp focus.

Britain, France, and Germany were all co-signatories of the 2015 deal limiting Iran’s nuclear program in exchange for sanctions relief. Since President Trump pulled out of the agreement last year, Britain and its EU partners have been trying hard to save it. But tightened U.S. sanctions on Tehran, the Iranians’ actions against oil shipping in the Gulf, and their initial move toward resuming uranium-enrichment have left the deal hanging by a thread.

This month, Britain did show signs of tilting toward the U.S. But only in part.

Royal Marines seized an Iranian oil tanker off Gibraltar – nominally because it was headed for Syria, which remains subject to EU sanctions, but reportedly on a tipoff from the U.S., which welcomed the move. Last Friday, however, Iran struck back. Despite warnings from a British Navy vessel, Revolutionary Guards boarded a British-flagged tanker and impounded it in the Iranian port of Bandar Abbas.

With only that one naval craft in the Gulf – and decades of budget cuts leaving Britain without a sufficient naval footprint to protect its shipping – the government was limited to threatening “serious consequences.”

Leery of joining U.S.

Until the tanker seizure, however, London had been leery of joining the Trump administration’s proposed arrangement for an international protection force, led by the U.S. and relying mostly on U.S. ships. Britain and its EU partners still hope to save the Iran nuclear deal, and above all prevent a military escalation with Iran. Britain was loath to ally itself with a more hawkish American policy.

In one of its final moves, Ms. May’s government still appeared to be hedging its bets. Foreign Secretary Jeremy Hunt – the candidate Mr. Johnson defeated for the top job – announced this week that while Britain was happy to “operate in partnership” with the Americans in the Gulf, he wanted a European­­-led international protection force, adding an explicit reaffirmation of British support for the Iran nuclear agreement.

Yet with Mr. Johnson publicly pledged to leave the EU by the end of October, a more definitive choice may have to be made between securing post-Brexit relations with major EU partners or seeking strengthened ties with the U.S.

In some ways, the issue of Britain’s international identity dates back to the end of its empire, after World War II. As late as the 1960s, former U.S. Secretary of State Dean Acheson pointedly remarked that the British had “lost an empire and not yet found a role.”

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Yet Britain remained a permanent member of the Security Council, and a major player in NATO. And in joining the European Economic Community, the precursor of the EU, in the 1970s, it found a significantly expanded role, not least as an often-indispensable connecting point between the U.S. and Europe.

If Mr. Johnson delivers on his promise to leave the EU within months, the main immediate challenge will be economic. But more broadly, as the Iran crisis has highlighted, a post-Brexit Britain will have to find new ways to exert influence and secure its interests on the world stage.