Emmanuel Macron and Donald Trump.

Carl P. Leubsdorf, Tribune News Service

Seventy-five years ago, thousands of American and allied sailors, soldiers and aviators braved heavy seas and murderous German fire in the historic invasion that began the decisive campaign to end Nazi control of Europe.

The success of the D-Day landings, Europe›s liberation 11 months later and its economic recovery over ensuing decades were a tribute not only to the bravery of the invading troops but to the unprecedented military and political unity that American presidents from Franklin Roosevelt to Barack Obama forged with European allies.

But as President Donald Trump and French President Emmanuel Macron mark Thursday›s D-Day anniversary at the Colleville-sur-Mer cemetery overlooking Omaha Beach, that unity is fraying, thanks to increasing nationalism within Europe and the tensions Trump has created by questioning the value of the continuing NATO alliance and clashing with long-time US allies over trade.

Even before becoming president, Trump questioned future US adherence to the way the North Atlantic Treaty Organization binds the United States to Europe by committing its signatories to aid one another if attacked. The 28-nation European Union was one of the early targets of Trump›s effort to revamp US trade agreements.

In addition, he has interfered to an unprecedented extent in internal European affairs, especially in Great Britain. He encouraged the Brexit movement that unexpectedly won a 2016 referendum calling for its separation from the EU, a vote still having repercussions today as the two parties struggle to adopt the divorce terms.

And on the eve of this week›s visit, Trump criticised Prime Minister Theresa May›s handling of the Brexit talks in an interview with a prominent British tabloid, the Sun, declaring she «allowed the European Union to have all the cards.» May is stepping down Friday because of her failure to reach a Brexit agreement.

But while thousands of protesters mocked Trump in the streets of London, the British sought to return the focus to the purpose of this week›s commemorations. At Monday night›s Buckingham Palace state dinner for the American president, Queen Elizabeth toasted the two nations› «close and lasting relationship» and gave Trump a copy of Winston Churchill›s World War II memoirs.

More significant than these personal flaps are the Trump policy moves that have convinced many European leaders they can no longer rely on the United States in the event of an emergency, among them his administration›s unilateral decisions to withdraw from the intermediate-range missile treaty with Russia and the multi-lateral Iran nuclear agreement.

For Macron, the breaking point with Trump came when the American president announced he was pulling US troops out of Syria. The two leaders are also at odds over the Iran agreement and US withdrawal from the 2015 Paris climate change pact. Trump has, when pressured, reiterated US commitment to supporting an attacked NATO ally. But he strained US relations with fellow members of the 70-year-old pact by repeatedly demanding that NATO members increase their defense spending, a point he reiterated in London.

While Trump has suffered increasingly rocky relations with Europe›s main US allies – Macron, May and German Chancellor Angela Merkel — he gave warm White House welcomes to their more authoritarian counterparts like Hungary›s Viktor Orban, who was shunned by the two previous US presidents. These displays by Trump, including his continuing initiatives toward North Korea›s Kim Jong Un and his reluctance to condemn Russian President Vladimir Putin›s aggressive moves in Ukraine, have raised questions in Europe whether there has been a fundamental change in US attitudes or this is unique to this president.