President Trump began the day by announcing unexpected tariffs, via tweet, on Brazil and Argentina. He'll end it in London, where fellow NATO leaders fear he could jolt the alliance once again over the next two days.

The big picture: Trump’s chaotic approach to foreign policy — tweets, threats, tariffs — leaves the world wary of news from Washington.

Driving the news: This week’s NATO gathering is designed as a chance for leaders to mark the alliance’s 70th birthday, with an abbreviated schedule to minimize the risk of flare-ups.

"All I'm hearing is great anxiety about what Trump might do or say," Ivo Daalder, a former U.S. ambassador to NATO, told Axios' Jonathan Swan.

Former national security adviser John Bolton recently remarked that Trump could withdraw the U.S. from NATO entirely if re-elected, according to NBC.

But it was French President Emmanuel Macron who drew headlines for questioning whether NATO was fit for purpose — with the U.S. pulling back and Turkey going rogue — in an interview with the Economist.

Turkish President Recep Tayyip Erdoğan responded with furious accusations of his own, while German Chancellor Angela Merkel scolded Macron for his characterization of NATO as brain dead.

Behind the scenes: Trump was also irked by Macron’s interview, Swan reports. "He's been down on Macron for a long while," one official told Swan, who reports that Trump has privately described Macron as a “wise guy.” Their "bromance" appears to be over.

What to watch: Trump’s relations with U.K. Prime Minister Boris Johnson are far warmer, but will also be tested this week.

With elections looming on Dec. 12 and opposition leader Jeremy Corbyn capitalizing on Trump’s past remarks and unpopularity in the U.K., Johnson will have to walk a perilous line between alienating Trump and appearing too close to him.

Brazilian President Jair Bolsonaro, meanwhile, has embraced Trump with both arms, but he was caught off guard by today's sanctions announcement.

Trump accused Brazil and Argentina of "massive devaluation of their currencies," though the slides are due to economic travails rather than government action.

Between the lines: There are domestic political considerations at play: Brazilian farmers are now supplying the bulk of China's soybeans as Trump's trade war hits U.S. farmers.

Zoom out: The dance practiced by the likes of Johnson and Bolsonaro — keeping Trump onside while bracing for his next remark — is all the more challenging for Ukrainian President Volodymyr Zelensky.

In a rare interview with TIME and three European outlets, Zelensky lamented Trump’s constant characterizations of Ukraine as corrupt — lobbed in the heat of an impeachment battle — as the “hardest of signals” to countries and companies from which Ukraine needs investment.

Zelensky also criticized Trump’s decision to withhold critical military aid. “If you’re our strategic partner, then you can’t go blocking anything for us. I think that’s just about fairness," he said.

The bottom line: Trump's partner one day can always become his target the next.

Go deeper: Bipartisan senators call for sanctions against Turkey before NATO summit