When news emerged in 2013 that the US government had monitored German Chancellor Angela Merkel's communications for years, Merkel sent a firm but conciliatory message to Washington. Friends don't “spy on each other,” she said, and then moved on to calm tensions.

At the time, US security experts defended the surveillance, saying friendships can easily end - an argument that was swiftly dismissed in Berlin, where strong transatlantic ties had long been the foundation of its international diplomacy.

Four years later, however, the once-unthinkable suddenly seems very real. President Donald Trump has repeatedly lashed out at Germany in recent weeks, taking to Twitter on Tuesday in the latest attack - targeting Germany's military spending and trade practices.

30 May tweet:

Germans, long opposed to many of Trump's campaign promises, are irritated and concerned by his criticism of their country. They wonder why the leader of Germany's most influential international ally seems more willing to criticise their country than nations with questionable human rights records.

“Europeans think they are now being treated worse by Trump than countries like Russia or Saudi Arabia,” said Stephan Bierling, an expert on transatlantic relations at the University of Regensburg in Germany.

Confronted with Trump's verbal attacks on Germany, Chancellor Angela Merkel - who is known for her cautious choice of words - appears to be increasingly outspoken about the ongoing decline of German-US relations. Although she was never a supporter of Trump, she initially expressed a willingness to collaborate with him if he respected values such as “the dignity of each and every person.” Her conditional offer of support, made the day after Trump's election, was meant as both an invitation to work together and a subtle warning.

But after contentious meetings with Trump last week, Merkel indicated changes in the US-German equation, saying Sunday that Europe “really must take our fate into our own hands” and emphasising that the days when her continent could rely on others was “over to a certain extent. This is what I have experienced in the last few days.”

In Berlin, Trump's attacks on Germany have raised questions about the future of the alliance. Besides strong economic ties, Germany hosts some of the United States' biggest military bases abroad and about half of all US soldiers on the continent. But Trump says Germany's trade surplus hurts US interests, and he has criticised what he deems as Germany's low defence spending. Merkel rejects most of his criticism as baseless.

The bilateral strains mean that the United States has, to some extent, lost the trust of one of Europe's most pro-American leaders. The German chancellor, the most powerful politician in Europe, grew up in Eastern Germany, and her upbringing there has long been credited for her staunch support for closer European-American ties. “Given her experience with the Cold War, Merkel has long upheld and defended American ideals. But the belief in shared values has been shattered by the Trump administration,” Bierling said.

As the German leader starts to focus on general elections in September and her bid for a fourth term in office, the transatlantic rift could further deepen. Leading Social Democrats said Monday that Merkel should have openly opposed Trump from the start rather than trying to work with him at first. “Merkel needs to put some distance between herself and Trump, who is exceptionally unpopular in Germany,” said Marcel Dirsus, a political scientist at the University of Kiel in northern Germany.

But how far is Merkel really willing to go? Dirsus cautioned that Germany remained reliant on the United States, especially in terms of its military. “Germany will have to cooperate with the US, whether it wants to or not,” he said.

In pictures: President Donald Trump on tour Show all 39 1 /39 In pictures: President Donald Trump on tour In pictures: President Donald Trump on tour 20 May 2017 US President Donald Trump and Saudi Arabia's King Salman bin Abdulaziz al-Saud arriving for a reception ahead of a banquet at Murabba Palace in Riyadh Getty Images In pictures: President Donald Trump on tour 20 May 2017 US President Donald Trump, accompanied by First Lady Melania Trump, and Saudi Arabia's King Salman bin Abdulaziz al-Saud being welcomed at Murabba Palace in Riyadh Getty Images In pictures: President Donald Trump on tour 20 May 2017 US President Donald J. Trump with King of Saudi Arabia Salman bin Abdulaziz Al Saud during a welcome ceremony with traditional sword dancers at Murabba Palace, in Riyadh, Saudi Arabia EPA In pictures: President Donald Trump on tour 20 May 2017 King of Saudi Arabia Salman bin Abdulaziz Al Saud with US President Donald J. Trump and wife Melania during a welcome ceremony at Murabba Palace, in Riyadh, Saudi Arabia EPA In pictures: President Donald Trump on tour 20 May 2017 US President Donald Trump adjusts the Collar of Abdulaziz Al Saud Medal, after it was bestowed upon him by Saudi Arabia's King Salman bin Abdulaziz Al Saud at the Royal Court in Riyadh, Saudi Arabia Reuters In pictures: President Donald Trump on tour 20 May 2017 Saudi Arabia's King Salman bin Abdulaziz Al Saud presents U.S. President Donald Trump with the Collar of Abdulaziz Al Saud Medal at the Royal Court in Riyadh, Saudi Arabia Reuters In pictures: President Donald Trump on tour 21 May 2017 Palestinians print posters depicting US President Donald Trump in preparations for his planned visit, in the West Bank town of Bethlehem Reuters In pictures: President Donald Trump on tour 21 May 2017 US President Donald Trump accompanied by first lady Melania Trump, his daughter Ivanka Trump and her husband White House senior advisor Jared Kushner, before delivering his remarks to the Arab Islamic American Summit in Riyadh, Saudi Arabia Reuters In pictures: President Donald Trump on tour 21 May 2017 US President Donald Trump looks on as U.S. Secretary of State Rex Tillerson and Saudi Arabia's Crown Prince Muhammad bin Nayef exchange a memorandum of understanding Reuters In pictures: President Donald Trump on tour 21 May 2017 First Lady Melania Trump shares a laugh with a child during a visit to the American International School in the Saudi capital Riyadh Getty Images In pictures: President Donald Trump on tour 21 May 2017 US President Donald Trump holds a bilateral meeting with Bahrain's King Hamad bin Isa Al Khalifa AP In pictures: President Donald Trump on tour 22 May 2017 Israeli soldiers rest during preparations ahead of President Trump's landing in Tel Aviv, Israel Getty Images In pictures: President Donald Trump on tour 22 May 2017 First Lady Melania Trump makes her way to board Air Force One in Riyadh as she heads with her husband the US President to Israel Getty Images In pictures: President Donald Trump on tour 22 May 2017 US President Donald Trump and first lady Melania Trump board Air Force One for Israel, the next stop in Trump's international tour, at King Khalid International Airport AP In pictures: President Donald Trump on tour 22 May 2017 Israeli soldiers wait for the arrival ceremony of US President Donald Trump at Ben Gurion airport near Tel Aviv, Israel AP In pictures: President Donald Trump on tour 22 May 2017 US President Donald Trump and first lady Melania Trump arrive aboard Air Force One at Ben Gurion International Airport in Lod near Tel Aviv, Israel Reuters In pictures: President Donald Trump on tour 22 May 2017 US President Donald Trump and first lady Melania Trump disembark Air Force One on arrival at Ben Gurion International Airport AP In pictures: President Donald Trump on tour 22 May 2017 US President Donald J. 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Trump, touches the Western Wall, Judaism's holiest prayer site, in Jerusalem's Old City EPA In pictures: President Donald Trump on tour 22 May 2017 President Donald Trump visits the Western Wall AP In pictures: President Donald Trump on tour 24 May 2017 US President Donald J. 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Trump EPA In pictures: President Donald Trump on tour 24 May 2017 Pope Francis gets into is car after meeting with US President Donald Trump AP In pictures: President Donald Trump on tour 24 May 2017 President Donald Trump and his wife Melania look at the frescoed ceilings during their visit to the Sistine Chapel at the Vatican L'Osservatore Romano/Pool Photo via AP In pictures: President Donald Trump on tour 24 May 2017 US President Donald Trump security vehicles are seen in front of Air Force One before take off from Leonardo da Vinci-Fiumicino Airport in Rome, Italy Reuters In pictures: President Donald Trump on tour 24 May 2017 US President Donald Trump and First Lady Melania Trump wave to reporters before boarding the Air Force One to Brussels, at the end of a 2-day visit to Italy including a meeting with Pope Francis at the Vatican, at Rome's Fiumicino international airport AP

Merkel, for instance, announced last spring that she wanted to increase Germany's annual defence budget by $27 billion over the next three years. That would almost double the current budget - but it would still be dwarfed by the $664 billion the United States spends every year.

Merkel is unlikely to warm to Trump as she did to former President Barack Obama. But she also won't be able to simply ignore him.