LONDON — What a difference four months can make.

When President Donald Trump assumed office on Jan. 20, European leaders braced themselves for upheaval. Emboldened by his unexpected victory last November, it seemed likely Trump was going to remold decades-old transatlantic relations. He called NATO obsolete, as well as the viability of the European Union. He provided succor to right-wing populist politicians who were also hoping to score electoral upsets. And he promised cozier relations with Vladimir Putin, the autocratic Russian leader intent on destabilizing Europe.

As America's allies prepare to welcome Trump on his first trip to Europe as president, the political calculus on both sides of the Atlantic has drastically changed. The American president arrives in Brussels on May 25 for a NATO summit, and will follow that with a two-day meeting in Italy the Group of Seven, the world's seven largest industrialized nations.

Nationalist politicians in the Netherlands and France fell short of expectations, and German Chancellor Angela Merkel looks like a shoo-in for re-election this fall. Meanwhile, Trump has apparently reversed his opinions about NATO and the EU, calling the former no longer obsolete and the latter "wonderful."

Trump also is increasingly occupied by ongoing investigations by the FBI and Congress into whether his campaign colluded with Russian efforts to interfere in last year's election on his behalf – probes that could imperil his presidency.

As a result, European leaders are "no longer as alarmed (by Trump) as they were last year," says Tim Oliver, an expert on European-American relations at the London School of Economics. They now realize that "Trump might be impulsive, but his words rarely lead to any substantive policies or challenges to the established order in NATO." Accordingly, expectations are low that the NATO summit will produce anything more than reassuring, boilerplate statements of amity.

But carefully worded platitudes will only serve to mask the reality that Trump's America is largely abdicating its NATO leadership role, weakening the once-durable military alliance. "Even if he commits to the alliance," says Sophia Besch, a research fellow at the Center for European Reform, "given the erratic comments of the past, U.S. leadership is still not there."

And, warns Oliver: "Without U.S. leadership, the alliance will stagnate."

Moreover, while the allies' fears have been somewhat assuaged by pro-NATO remarks by other top administration officials, particularly Defense Secretary James Mattis, they also know there's no guarantee that Trump won't change course again. There are some worrying signs. Senior policy adviser Stephen Miller, a NATO skeptic, is writing the speech Trump will deliver in Brussels, and a top aide last week warned that Trump may yet decide to pull the U.S. out of NATO.

Nevertheless, "everyone hopes he says the right things, they want reassurances," says Pierre Vimont, a senior fellow at Carnegie Europe, a Brussels think tank, and a former French ambassador to both the EU and the U.S. "But you don't regain trust in five minutes. They will watch for further actions. It will take some time."

Trump is focused on two issues. He wants NATO more involved in counterterrorism, and particularly wants it to join the coalition fighting ISIS in Syria and Iraq. Although all 28 of NATO's member states are already part of the coalition, Trump wants the organization itself to become involved.

The U.S. president also wants NATO countries to keep to their commitment to spend 2 percent of their gross domestic product on defense. NATO will likely agree to join the coalition, though not in a combat role. And Vimont says members are slowly inching toward meeting their funding targets. "It's moving in the right direction." But it remains to be seen if it's enough to placate Trump.

European leaders will want Trump to assure them that America still supports Article 5 of the NATO treaty, which states that an attack on one member is an attack on them all. That point has become particularly germane because of heightening fears that Russia could invade one of the Baltic States. Mattis and other administration officials have restated America's commitment to Article 5, but Trump hasn't. "Article 5 is No. 1 to European leaders," Besch says.

Europeans are keenly aware of Russian efforts to destabilize the continent and create rifts in NATO, says Richard Whitman, a professor of politics and international relations at the University of Kent. "Part of the glue that holds the EU together is a shared anxiety over Russia."

Which is why they're apprehensive about Trump's pro-Russian sentiments and his alleged campaign's links to Russia. Nonetheless, Besch says, it's doubtful they'll raise those concerns this week. "They will not want to make it a topic and risk falling out over it."

At the G-7 summit, which takes place on the island of Sicily, the other six leaders will hope to get a better sense of Trump's current thinking on several issues. There are worries about Trump's protectionist views on trade, his skepticism about the 2015 international agreement that prevents Iran from developing nuclear weapons and his threat to pull America out of the Paris climate accord.

And at both summits, Trump's counterparts will be only all too aware that they are dealing with a man whose chaos-plagued presidency is already struggling.

"I don't think the allies will allow that to enter their thinking," Besch says.