When news emerged in 2013 that the U.S. 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, U.S. security experts defended the surveillance, saying friendships can easily end - an argument that was swiftly dismissed in Berlin, where strong trans-Atlantic 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. May 30 tweet:

"We have a MASSIVE trade deficit with Germany, plus they pay FAR LESS than they should on NATO & military. Very bad for U.S. This will change."

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 criticize 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 trans-Atlantic 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-U.S. 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 U.S.-German equation, saying Sunday that Europe "really must take our fate into our own hands" and emphasizing 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 U.S. soldiers on the continent. But Trump says Germany's trade surplus hurts U.S. interests, and he has criticized what he deems as Germany's low defense 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 trans-Atlantic 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 U.S., whether it wants to or not," he said.

Merkel, for instance, announced last spring that she wanted to increase Germany's annual defense 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.