No sooner had Trump touched down in Paris for a weekend of commemorations to mark the end of World War 1, than he fired off a tweet castigating his host over proposals to endow the EU with its own, joint army.
"President Macron of France has just suggested that Europe build its own military in order to protect itself from the US, China and Russia," the US president tweeted, referring to remarks made by Macron three days earlier.
"Very insulting, but perhaps Europe should first pay its fair share of NATO, which the US subsidizes greatly!," he added.
Trump, who is visiting France with his wife Melania for the second time since becoming president, was referring to a call made by Macron in an interview Tuesday for a "real European army."
Macron, an ardent advocate of closer European integration, said a joint European Union military force was needed to wean Europe off American might, not least after Trump announced he was pulling out of a Cold War-era nuclear treaty.
"We have to protect ourselves with respect to China, Russia and even the United States," he told Europe 1 broadcaster, suggesting for the first time that Europe might need to defend itself from America.
The French president, who has enacted major defence spending hikes to bring France in line with NATO spending targets, is spearheading the creation of a nine-country European rapid reaction force, independent from NATO.
The force, which would fall far short of an army in size and scope, would be able to rapidly mount a joint military operation, evacuate civilians from a war zone or provide aid after a natural disaster.
Proposals for a full EU army with a joint command — a pet project of European federalists — remain deeply sensitive, however, among EU members anxious to defend their sovereignty.
French officials said Macron's mooted EU "army" was merely a call for closer defence integration.
Bilateral talks, then lunch
The row over EU defence risks overshadowing commemorations to be attended by 70 world leaders, including Russian President Vladimir Putin and Turkey's Recep Tayyip Erdogan.
On Saturday morning, Macron and Trump will hold bilateral talks on issues expected to include Syria and Iran before lunching with Melania and Macron's wife Brigitte.
The last time the Macrons hosted the Trumps in Paris in July 2017, during Bastille Day celebrations, they whisked them up the Eiffel Tower for a private dinner on the second floor.
The tone of this visit will be markedly less festive. Trump is scheduled to visit two American cemeteries over the course of the weekend, which will culminate with a solemn ceremony Sunday at the Tomb of the Unknown Soldier under the Arc de Triomphe, a monument to France's war dead.
Speaking to reporters Friday before his departure Trump said he was "looking forward" to the trip."I think it will be something very, very special," he said.
The isolationist US leader has, however, ducked out of a peace conference on Sunday afternoon, which Macron and Chancellor Angela Merkel intend to use as a platform for promoting multilateralism.
US National Security Advisor John Bolton Friday downplayed Trump's decision not to attend the inaugural Paris Peace Forum — to be attended by Putin and Erdogan — telling reporters in Paris the president had "a lot of pressing issues" to attend to.
Trump and Macron struck up a warm relationship initially, particularly during the US leader's first visit to Paris, but have repeatedly clashed since then over a growing list of issues, including Trump pulling America out of the 2015 Paris climate accord and the Iran nuclear deal.
Meeting in forest clearing
The WWI commemorations come at a watershed moment for the liberal post-war order, with anti-immigration populists at the helm in the US and Brazil, sharing power in Italy, and making strong gains in Germany, prompting Merkel to announce she is bowing out in 2021.
Macron met UK Prime Minister Theresa May near the Belgian border Friday to try to make progress on a Brexit deal and remember the fallen of the battlefields of the Somme.
On Saturday afternoon, he will visit the northern town of Compiegne where the armistice ending World War 1 was signed by the Allies and Germany in a train carriage on November 11, 1918.
Macron will meet German Chancellor Angela Merkel at the site, in a visit symbolising the close ties between two countries that fought three wars between 1870 and 1945.