Germany and Europe must prepare for Donald Trump to make the wrong decision on the Iran deal. It may be up to Europe to prevent the worst after May 12.

On the dark side.

Germans are hearing faint echoes of 2003. Back then, during another low point in US-German relations, the Americans rushed into a confrontation in the Middle East based on intelligence about weapons programs that later turned out to be false. The Germans and French parted ways with the US (although Britain, Spain and Poland joined the American alliance). The ensuing conflagrations along the Tigris and Euphrates were human disasters. German-American relations took years to recover.

Something similar could unfold again after May 12. That is when US President Donald Trump is expected to re-impose sanctions on Iran, thereby pulling out of the Joint Comprehensive Plan of Action (JCPOA), better known as the Iran nuclear deal.

Brokered largely by the Europeans in 2015, this deal was neither seen nor sold as perfect. It does not address Iran’s efforts to build ever fancier ballistic missiles, nor its meddling in Syria, Iraq, Yemen and elsewhere. Instead, it was meant to prevent Iran from getting the weapons-grade uranium and plutonium to build nuclear bombs, under the constant monitoring of international auditors. At that, the JCPOA appears to be succeeding.

But Mr. Trump keeps calling this arrangement a “terrible deal.” On April 30, Israel’s prime minister, Benjamin Netanyahu -- channeling the hapless Colin Powell in 2003 -- gave a presentation about Iran’s past nuclear programs, implying that Iran must be breaching the 2015 deal in all sorts of ways even today. But the International Atomic Energy Agency says Iran is complying. European spy agencies agree. This time Britain, France and Germany are united in wanting to save the Iran deal, and thus united in being ready to oppose Donald Trump.

So what happens when Trump does pull out? In the worst scenario, the hardliners in Iran would see themselves vindicated and push aside the moderates. Iran would then sprint for a “breakout” and actually build nukes. Saudi Arabia, Iran’s bitter enemy, would start its own nuclear program. The Israelis and Americans would launch preemptive attacks on Iran, igniting another conflagration. Hundreds of thousands would die, suffer or flee to Europe.

This time Britain, France and Germany are united in being ready to oppose Donald Trump.

North Korea, meanwhile, where Kim Jong-un is making all sorts of uncharacteristically conciliatory gestures about his own nuclear program, would conclude that making concessions to America such as Iran’s in 2015 would be -- how else to put it -- a “terrible deal.”

This is why Europe -- and especially its most populous nation, Germany, that sleeping giant of world diplomacy -- must now step up and lead the West as America apparently no longer wants to do. The Europeans should offer immediate economic relief to Iran after Trump’s pull-out, argues Mark Leonard of the European Council on Foreign Relations. Oliver Meier of the German Institute for International and Security Affairs goes even further: He wants Europe to invoke its so-called “blocking statute” against America as soon as the US imposes secondary sanctions against European firms still doing business with Iran.

The message to Angela Merkel, Emmanuel Macron, and Theresa May should be clear. Save the Iran deal in some form. Keep the international monitors inside Iran. Keep trading and talking with the pariah state. Do all this to help the moderates inside Iran guide their country away from the paths that lead to war, and toward a tolerable coexistence in the region and world.

Andreas Kluth is editor-in-chief of Handelsblatt Global. You can reach him at [email protected]