World leaders who thought they'd seen the back of former President Trump are now bracing for the uncomfortable prospect of a second Trump presidency.

Why it matters: U.S. allies had never seen a president like Trump, who threatened to rip up treaties and often made bilateral disagreements personal. While long-serving leaders may have known Trump's return was possible, it's now something they have to actively prepare for.

Driving the news: Just yesterday, Trump said at a rally in Conway, S.C., that he told the president of an unidentified NATO member he would "encourage" Russia "to do whatever the hell they want" if the NATO member were behind on its obligations to the alliance.

• "One of the presidents of a big country stood up and said: 'Well, sir, uh, if we don't pay and we're attacked by Russia, will you protect us?" Trump recounted.
• "You didn't pay? You're delinquent?" Trump said he replied. "No, I would not protect you. In fact, I would encourage them to do whatever the hell they want.' You gotta pay! You gotta pay your bills."
• White House spokesman Andrew Bates responded: "Encouraging invasions of our closest allies by murderous regimes is appalling and unhinged."

President Biden recently claimed that world leaders kept pulling him aside with a message: "You've got to win. We can't let [Trump] happen again."

• While Biden's remarks can't be verified, a handful of prominent global leaders have expressed their concerns openly.

What they're saying: Canadian Prime Minister Justin Trudeau said that his government was preparing for the "uncertainty" Trump's return would bring.

• "We made it through" four years of Trump, said Trudeau — with whom Trump clashed over trade and condemned variously as "weak" and "two-faced" — adding that it had been "difficult."
• German Chancellor Olaf Scholz risked Trump's wrath by openly endorsing Biden last year over the "division" he said Trump represents. Scholz took office after Trump left the White House, but his predecessor Angela Merkel was a frequent target of Trump's antipathy.
• European Central Bank Chief Christine Lagarde cited Trump's penchant for imposing tariffs on allies, his shaky commitment to NATO and his opposition to climate action when she labeled his political resurgence a "threat" to Europe in January.

French President Emmanuel Macron was more diplomatic last month, saying "I take the leaders that people give me."

• Flashback: Macron tried a bear hug strategy with Trump early in his tenure but relations deteriorated over time.
• An additional twist came after Trump's defeat, when the FBI seized a document about the "President of France" during the 2022 raid of Mar-a-Lago. While no specifics have emerged, Rolling Stone reported separately that Trump had boasted to associates of having intel on Macron's sex life.

Perhaps no leader has more to fear from Trump's return than Ukrainian President Volodymyr Zelensky.

• Trump's allies in Congress have held up additional military aid to Ukraine, and the former president has said he'd end the nation's war with Russia "within 24 hours" of taking office — an unsubstantiated claim Zelensky called "very dangerous."
• Another sticking point: Trump was impeached for pressuring Zelensky to dig up dirt on Biden.

Outside of Europe, a Trump return could spark concerns in South Korea, due to Trump's past threats to withdraw troops and impose tariffs, as well as Taiwan.

• Trump answered a recent question about whether he'd protect the self-governing island from China by stating that Taiwan took "all of our chip business."

The flipside: A handful of still-sitting leaders, such as Hungarian Prime Minister Viktor Orbán and Polish President Andrzej Duda, established warm relations with Trump during his term.

• While Trump-friendly former Brazilian President Jair Bolsonaro is gone, Trump celebrated the victory of self-described "anarcho-capitalist" Javier Milei in neighboring Argentina last November.

The intrigue: Israeli Prime Minister Benjamin Netanyahu may be among the leaders hoping for a Trump return, given their close political alliance and the international pressure Netanyahu and Israel are now under.

• But Trump was not impressed when Netanyahu congratulated Biden for his election victory, fuming in a 2021 interview with Axios' Barak Ravid: "F**k him."

Between the lines: Trump's resurgence hasn't taken the world entirely by surprise.

• Ever since he left office, conversations with foreign diplomats in Washington inevitably turned to the question of whether he could come back.

The bottom line: Speculation about a Trump return has now turned to preparation.