Trump's abrupt announcement that he will reduce US troops in Germany by half to 25,000 -- a decision that has brought concern across NATO -- is the culmination of mounting tensions between the leaders of the Western alliance's two most populous nations.

Trump called Germany a "delinquent" to NATO -- a reference to its failure to meet a target of spending two percent of GDP on defense -- and said, "they treat us very badly on trade."

The latest episode came after Merkel, a scientist before entering politics who acted early on the coronavirus pandemic, snubbed Trump's plan to convene the Group of Seven leaders in Washington this month.

Trump postponed the summit -- and said he would expand it to other leaders, including Russian President Vladimir Putin, who was kicked out over the takeover of Crimea.

While Merkel's spokesperson cited public health concerns, Trump had been hoping to show a return to normal in the United States as he seeks a second term in November 3 elections.

- No patience for Trump -

Trump's 2016 election shocked US allies but most tried to deal with him. Japanese, British and French leaders all flattered Trump with invitations, even if French President Emmanuel Macron was also vocal on disagreements over issues from climate change to Iran.

Merkel from the start did little to hide her disdain for Trump, who ran on a platform of closing US borders and had explicitly criticized Germany's welcome to millions of migrants.

Several months after Trump took office, Merkel made waves when she said that the United States under Trump and Britain, which voted to leave the EU, were no longer reliable partners and that Europe should "take its fate into its own hands."

Trump in turn shattered norms of polite behavior between allies. In 2018, he wrote on Twitter that Germans were "turning against their leadership" over the "big mistake" on immigration and incorrectly said that crime was "way up" in Germany.

Trump -- himself of German ancestry -- has frequently clashed with powerful women, taking sharply personal tones with domestic rivals including Hillary Clinton and Nancy Pelosi.

Sudha David-Wilp, a senior transatlantic fellow at the German Marshall Fund of the United States, said that while gender could be a factor, Merkel had also been "joined at the hip" with Trump's predecessor Barack Obama toward the end of his presidency.

For Obama, "Germany was seen as the indispensable partner, especially in light of Brexit," David-Wilp said.

"So I also think President Trump of course was probably wary of Angela Merkel and the other way around," she added.

And in personality, "President Trump and Chancellor Merkel are diametrically opposite," she said, with Trump likely realizing immediately that she had no patience for attempts to charm her.

David-Wilp noted that Trump's talk of Germany "free-riding" on US security is not new, with the real estate mogul criticizing the major auto exporter as far back as the 1980s.

- Better under Biden? -

David-Wilp said that US-German relations may improve if Trump loses to presumptive Democratic nominee Joe Biden, who was vice president under Obama.

But she noted that US-German differences, if expressed more subtly, were not absent under Obama, who had faulted Germany for not doing more for the whole of the European Union.

Germany, which took a harsh stance during Greece's economic crisis, has recently shown a softer side in supporting a post-pandemic relief package for the continent.

"The issues are longstanding and they are not going to go away. But under a Biden administration, there will certainly be an effort to repair the damage that's been done," David-Wilp said.

Ivan Krastev, chairman of the Centre for Liberal Strategies, a research group based in Bulgaria, told a conference at the Brookings Institution that the mood had appeared to shift in Germany, "once the most pro-Atlantic country."

He believed that opinions could change. But he said that Europeans, even if they liked Obama, did not entirely approve of his policies.

"I do believe people are going to make a mistake if they believe that simply because Biden is back, Europe is back in its relations with the United States."