By Noah Barkin
BERLIN (Reuters) - Following Donald Trump's decision to decertify the Iran nuclear deal this month, shaken European capitals are asking themselves what the U.S. president will do next.
Diplomats have sketched scenarios they fear could plunge transatlantic relations into crisis: a trade war, military conflict over North Korea, or the collapse of a Cold War-era arms treaty. They wonder if their post-war alliance can survive if any of them come true.
Since Trump took office nine months ago, governments in Berlin, Paris and London have wavered between alarm at his rhetoric and a shaky sense that his worst instincts can be contained by the "grown ups" in his cabinet and pressure from allies.
But the Iran decision, taken despite personal appeals from France's Emmanuel Macron, Germany's Angela Merkel and Britain's Theresa May, has changed the calculus in Europe, according to diplomats, politicians and analysts.
No longer is there an underlying confidence that Europe can muddle through three more years of Trump without fear of major, and possibly lasting, disruptions to the relationship. Nor is there faith that Trump, when the stakes are high, will listen to what his advisers and partners tell him.
The anxiety is especially acute in Germany, whose own identity is anchored in its relationship with the United States and whose reliance on Washington, particularly in defense and security matters, runs deeper than that of France or Britain.
"There is a sense of desperation in Berlin, a sense that Trump does not know what is at stake, that he doesn't understand the historical factors that are at play here," said Wolfgang Ischinger, a former German ambassador to Washington and chairman of the Munich Security Conference.
"The transatlantic relationship is all about trust. In that sense, the Iran decision takes us to a new level. It is a breach of trust," he said.
STANDING IN THE RUBBLE
Trump's decision to decertify the Iran deal will not necessarily kill it. That depends on Congress, which must now decide to impose new sanctions against Tehran. But the consensus in Europe is that a landmark diplomatic achievement has been seriously undermined.
Merkel alluded to the breakdown in trust back in May, when she said Europe might not be able to rely on the United States and must take its fate into its own hands.
That came after a contentious G7 meeting in Sicily at which Trump ignored pleas from allies to stick with another multilateral agreement, the Paris climate accord.
Since then Merkel, focused on an election and efforts to form a coalition, has said little about the transatlantic relationship or Trump.
Macron meanwhile, has done his best to charm the American president, inviting him to Paris for Bastille Day celebrations and meeting him again at the United Nations last month.
"I don't despair of making him change his mind," Macron said in New York of Trump's stances on climate and Iran.
In private however, European officials say they are worried that Trump's rejection of the Iran deal could be a harbinger of other disruptive salvos from Washington.
One senior European diplomat said the next conflict would likely be over trade, describing Trump's attempt to renegotiate the North American Free Trade Agreement (NAFTA) -- and his threats to abandon it -- as a "litmus test".
European firms, especially German carmakers who produce in Mexico and export to the United States, could be hit hard by a breakdown of NAFTA.
There are also fears that Trump could carry out a threat to introduce steel import tariffs, which would hit European exporters as well as the Chinese.
"So far there has been a lot of bark and no bite from Trump on trade, but that doesn't mean it will stay that way," said the European diplomat. "We have to be prepared for real protectionist measures. Any steps that penalize European firms directly or indirectly would create a downward spiral."
North Korea is another area of concern. Thorsten Benner, director of the Global Public Policy Institute in Berlin, believes the reaction in Europe to a military conflict on the Korean peninsula could be "as messy as the Iraq war".
The European public could view Washington as the aggressor in such a conflict. This could fuel anti-American sentiment, making it difficult for European leaders to stand with Trump and emboldening voices that are already calling for a break from Washington -- no matter how unrealistic that looks in practice.
"We have to wake up to the fact that we may be dealing with Trump for some time, and that there may not be a correction when he leaves," one German diplomat told Reuters. "It hasn't dawned on people here that this is existential and you have to react."
Concerns about such sentiments led a group of a dozen Germany-based foreign policy experts to issue a manifesto this month, entitled "In Spite of It All, America", which warned Berlin against turning its back on Washington because of Trump.
Former ambassador Ischinger shares that view. But he too is concerned the relationship could deteriorate further.
One of his biggest fears is a collapse of the INF Treaty, a nuclear arms reduction pact signed by President Ronald Reagan and Soviet leader Mikhail Gorbachev in 1987. Washington and Moscow accuse each other of violating the treaty and Gorbachev himself warned this month that it was in peril.
Should the INF unravel, Ischinger believes the divisive 1980s debate about deploying nuclear weapons in Europe could resurface with devastating consequences for transatlantic ties.
"All hell could break loose, politically speaking," he said.
For generations of Germans, Ischinger says, the symbol of the transatlantic alliance has been U.S. presidents -- from John F. Kennedy to Reagan, and from Bill Clinton to Barack Obama.
"I can no longer tell my children that they are part of this alliance with Trump as president," he said. "It is rather sad. I don't know how the loss of this symbol can be replaced."
(Writing by Noah Barkin; Additional reporting by John Irish in Paris and Robin Emmott in Brussels; Editing by Giles Elgood)