BRUSSELS – Facing the possibility of a fundamental disengagement under U.S. President Donald Trump, European Union leaders opened a day of emergency summit talks Thursday to beef up their own military defenses and make sure that Ukraine will still be properly protected by its allies.

Friedrich Merz, the likely next chancellor of Germany, was conferring with summit host Antonio Costa over breakfast on how to meet the challenge on a short deadline only days after he and his prospective coalition partner pushed plans to loosen the nation’s rules on running up debt to allow for higher defense spending.

At the same time, the 27-nation bloc was waking up to the news from French President Emmanuel Macron would confer with EU leaders the possibility of using France’s nuclear deterrent to protect the continent from Russian threats.

It all underscored the sea change that has happened in the two months since Trump took office and immediately started questioning the cornerstones of cooperation between the United States and Europe which had been the bedrock of Western security since World War II.

“Given these profound shifts in US policy, and the existential threat of another war on the continent, Europe, must manage its essential defence tasks,” the European Policy Center think tank said in a commentary.

The bloc of 27 will “take decisive steps forward,” Macron told the French nation Wednesday evening. “Member states will be able to increase their military spending” and “massive joint funding will be provided to buy and produce some of the most innovative munitions, tanks, weapons and equipment in Europe.”

Adding to the ebullient message he said that “Europe’s future does not have to be decided in Washington or Moscow.”

European Commission President Ursula von der Leyen wants to be up to the task and has proposed an 800 billion euro ($840 billion) plan that would allow EU member states to spend much more on defense despite their current budgetary woes and profit from loans to kickstart the process.

Part of any plan is also to protect the increasingly beleaguered position of Ukraine, and President Volodymyr Zelenskyy is set to take part in the talks.

Early this week, Trump ordered a pause to U.S. military supplies to Ukraine as he sought to press Zelenskyy to engage in negotiations to end the war with Russia, bringing fresh urgency to the EU summit in Brussels.

“Europe faces a clear and present danger on a scale that none of us have seen in our adult lifetime. Some of our fundamental assumptions are being undermined to their very core,” European Commission President Ursula von der Leyen warned in a letter to the EU’s 27 leaders, who will consider ways to access more money for defense spending and ease restrictions on it.

But perhaps the biggest challenge for the EU on Thursday will be to take a united stance at a moment when it’s fractured, since much of what the bloc does requires unanimous support.

Even if the challenges are so daunting, Thursday’s summit is unlikely to produce immediate decisions on spending for Ukraine or its own defenses. Another EU summit where the real contours of decisions would be much clearer is set for March 20-21.

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AP journalist Sylvie Corbet contributed to this report.