This past July, on the final day of the NATO summit in Brussels, Angela Merkel, the German Chancellor, proposed a closed-door emergency meeting. The emergency was Donald Trump. Minutes earlier, the President had arrived late to a session where the Presidents of Ukraine and Georgia were making their case to join NATO. Trump interrupted their presentation and unleashed a verbal assault on the members of the alliance, calling them deadbeats and free riders on American power. Trump threatened to go his “own way” if they didn’t immediately pay more for their own defense. His barrage centered on Merkel, Europe’s longest-serving democratic leader.

“You, Angela,” Trump chided Merkel. Most of NATO’s members had failed to fulfill the goal of spending two per cent of G.D.P. for defense, but Trump focussed on Germany’s military spending of just over one per cent of G.D.P. In front of television cameras the previous day, he had accused Germany of being “totally controlled by Russia,” because of a proposed new gas pipeline. His tweets that day sounded like blackmail. “What good is NATO if Germany is paying Russia billions of dollars for gas and energy? The U.S. is paying for Europe’s protection, then loses billions on Trade. Must pay 2% of GDP IMMEDIATELY, not by 2025.”

Now, with the room cleared of staff, the NATO leaders sat stunned by what one called the “bizarre spectacle” of Trump’s harangue. It fell to another woman, Dalia Grybauskaitė, the President of Lithuania, to defend Merkel. Germany had sent troops to protect Lithuania from Russia, Grybauskaitė pointed out, and Merkel was committed to spending more on NATO’s common defense. The Danish and the Norwegian Prime Ministers also pushed back. In the corner of the room, Merkel strategized with other Europeans about how to stop Trump. Eventually, Mark Rutte, the Prime Minister of the Netherlands, offered the President what he appeared to want most: a way to claim victory. Rutte noted that, since Trump took office, the NATO allies had collectively raised their defense budgets by some seventy billion dollars. Take the win, he urged the President. Trump did just that.

And yet, when he emerged from the meeting and spoke with reporters, Trump lied, claiming not only that his allies had capitulated to him but also that they would consider his demand to raise their annual military spending to four per cent of G.D.P., an assertion so politically impossible that Emmanuel Macron, the President of France, immediately issued a public rebuttal. Trump, of course, went on behaving in his erratic, inexplicable manner. As he left the summit, he interrupted the Chancellor while she was addressing her fellow NATO leaders, and kissed her. “I love this woman,” he said. “Isn’t she great?” A senior German official who told me about that particular Trumpian flourish resisted any attempt at full understanding. “It’s up to psychologists and historians what to make of that,” he said. Four days later, Trump ended his European tour in Helsinki. There, standing next to Vladimir Putin, he spoke with bewildering sympathy for Russian foreign policy, ill-concealed contempt for his NATO partners, and implausible skepticism about his own intelligence services.

The astonishing week in Europe was the culmination of a four-month period in which the President imposed tariffs on allies by calling them “national security” threats, pulled out of the Iran nuclear deal, which they had jointly negotiated with the United States, and held affectionate summits with two of the world’s most powerful autocrats, Putin and Kim Jong Un, while casting aspersions on America’s most constant allies. No one was more of a constant target than Angela Merkel.

European leaders now worry that Trump’s illiberal aims go well beyond his insistent demands on Merkel to pay more for NATO and stop shipping so many cars to the U.S. “Many European leaders have told me that they are convinced that President Trump is determined to destroy the E.U.,” a former senior U.S. official told me. Trump has begun publicly calling the E.U. a “foe,” and promoting the resurgence of nationalism, which Macron and Merkel see as a direct threat. Trump’s Secretary of State, Mike Pompeo, in a recent speech at the German Marshall Fund in Brussels, attacked the United Nations, the E.U., the World Bank, and the International Monetary Fund, and derided what he called Europe’s flawed vision of multilateralism as “an end to itself.”

Europe has had many fights with American Presidents over the years, but never in the seven decades since the end of the Second World War has it confronted one so openly hostile to its core institutions. Since Trump’s election, Europe’s leaders have feared that it would come to this, but they have disagreed about how to respond to him. Many hoped to wait Trump out. A few urged confrontation. Others, especially in nations more vulnerable to Russia, urged accommodation. (Poland offered to name a new military base Fort Trump.) Macron tried flattery, and then, when that failed, he reverted to public criticism of Trump-style nationalism.

The challenge from Trump has been especially personal for Germans, whose close relationship with the United States has defined their nation’s postwar renaissance. Merkel grew up in Communist East Germany and credits the United States as essential to the liberation of the East and to German reunification. As the head of Europe’s largest and wealthiest nation, she has sought to guide the Continent through the standoff with Trump, but has struggled, because the President’s harsh words reflect a painful truth: Europeans are dependent on the United States for their security and increasingly divided as Putin’s Russia threatens the nations in the east. “Not all of what he says is wrong,” said the senior German official, one of ten who spoke with me. “Europe has been free-riding for some time.” Asked for comment about Trump’s criticism of Merkel, a White House spokesperson told me, “He is often toughest on his friends, and he considers her one. He views Germany as a powerful, prosperous country that should be doing more on defense spending.” But the risks for Trump are also considerable: call your friends enemies long enough, and eventually they may start to believe you. Is this, then, finally, the end of Pax Americana?

On November 16, 2016, eight days after Trump was elected, Barack Obama flew to Berlin to meet with Merkel; it was the last foreign trip of his Presidency. Obama and Merkel had not started out as good friends, but they had become as close as two public figures could be. Over dinner in the Adlon Hotel, they discussed the shocking events of the previous few months, particularly Great Britain’s referendum to leave the European Union and Trump’s victory running on the slogan “America First.” Through the windows of their private dining room, Merkel and Obama could see the floodlit Brandenburg Gate, the symbol of a reunified Berlin.

Merkel, who was nearing the end of her third term, confided that she reluctantly felt that she had to run again, in order to be a buffer against Trump, Brexit, and the surge of right-wing populism throughout Europe. Obama urged her to do so. “Obama was obsessed with the fate of Europe during his last year in office,” Charles Kupchan, who served as Obama’s top National Security Council adviser on European affairs and accompanied him on the trip, told me. After the election, the situation seemed even more urgent. “His view was that Merkel was needed to keep Europe together,” Kupchan said. “He was afraid that, without Merkel, Humpty Dumpty was going to fall off the wall.”