by Michael Birnbaum  and Anthony Faiola

BRUSSELS — President Trump arrived in Belgium on Wednesday for an audience with the nation's king, a day ahead of meetings   with leaders of alliances he once derided as irrelevant — and many top officials here say they will count it a success if there are no blowups during the visit.

From a European Union hit by populists to a NATO filled with concern over U.S. security guarantees, the city in which the U.S. president touched down Wednesday will be on its best behavior over the course of his 29-hour visit. The mere fact that Trump has agreed to visit a city filled with international organizations he once called "obsolete" is a victory, some here say.

E.U. leaders plan a simple meet-and-greet on Thursday morning, with a focus on "connecting the synapses" about a handful of European priorities such as trade and security, according to a senior E.U. official involved in the planning. France's new president, Emmanuel Macron, will press Trump on U.S. environmental commitments over lunch. 

NATO leaders have arranged an itinerary to appeal to the former real estate magnate: a ribbon-cutting of the alliance's glassy new headquarters, followed by a dinner where leaders will be held to a lightning-round speaking schedule to save time.

And at a summit of the leaders of the Group of Seven world powers in Sicily on Friday and Saturday, Trump will be pressed to reaffirm the U.S. commitment to free trade and to keep the United States in the Paris agreement on climate change. The White House has said a decision on the climate deal will come shortly after the visit.

Trump plans to press NATO leaders on defense spending, continuing a line of attack he started as a candidate last year, Secretary of State Rex Tillerson said Wednesday.

"You can expect the president to be very tough on them," Tillerson said, saying that he expected Trump to tell them: "The American people are doing a lot for your security, for our joint security. You need to make sure you're doing your share for your own security as well."

Although Trump had initially been expected to make an announcement on Afghan troop levels at the NATO meeting, Tillerson said that a policy review has not been completed and will take at least a few weeks.

At the G-7 meeting, Tillerson said that no trade deal was in the offing. Instead, he said, European leaders should expect "a very frank discussion and exchange on why these trade unbalances exist."

Endorsing an Obama-era position on Russia's exclusion from the group, the secretary of state said that the Kremlin would have to ensure progress on peace in eastern Ukraine and restore "Ukrainian sovereignty" over its territory before it could return.

Trump's meetings with European leaders come after months of anxiety from nations that for generations have been the United States' closest partners but that the president has sometimes appeared to view as free riders. Worries have calmed since their January heights. But many Europeans say they still are unsure what to expect Thursday.

"There's still a high degree of uncertainty when it comes to the aims and objectives of the Americans," said Cornelius Adebahr, an associate fellow at the German Council on Foreign Relations. "One of the main objectives is to convince the Americans of the value of these formats."

With Russia-related chaos enveloping Washington and Trump sharing sensitive intelligence with the Kremlin that he has apparently held back from European allies, many here say their confidence is fragile.

Nervous NATO members also hope to hear Trump's personal commitment to the alliance's security guarantees after he called them into question on the campaign trail.

Trump is "someone who doesn't believe in the whole idea of engaging with European allies," said Tomas Valasek, who until April was the Slovakian ambassador to NATO and is now the head of the Carnegie Europe think tank. "At least part of the European countries' strategy for dealing with Trump is essentially to hunker down and wait until he goes away."

Even for the brief meetings on Thursday, significant effort was being exerted on how to communicate with Trump, officials said.  

At Trump's hour-long morning meeting with European Council President Donald Tusk and European Commission President Jean-Claude Juncker, the two European leaders plan to push Trump to endorse free trade and will highlight the work that European nations are doing to fight terrorism, according to a senior E.U. official who spoke on the condition of anonymity to discuss internal planning. The message gained more urgency after Monday's Islamic State-claimed terrorist attack in Manchester. 

At NATO, leaders are being coached to keep their statements crisp, muscular and under four minutes, a standard time limit for such meetings that is being enforced with special vigor this year to avoid upsetting the notoriously impatient U.S. president. 

Trump will christen the alliance's new headquarters, dedicating a memorial that includes a fragment of Manhattan's destroyed World Trade Center. The only time NATO's collective defense pledge has been invoked was by the United States after the terrorist attacks of Sept. 11, 2001, which resulted in the years-long NATO operation in Afghanistan. 

The meeting will deliver a "strong message of unity and solidarity," NATO Secretary General Jens Stoltenberg said Wednesday ahead of the gathering.

At the meeting, leaders are expected to agree to create plans by the end of the year to achieve NATO defense spending goals, a spur for the 23 of the alliance's 28 countries that do not currently meet the target.

Trump's 2018 budget proposal, unveiled Tuesday, would increase U.S. military spending in Europe by $1.4 billion, a 41 percent bump that is a departure from earlier warnings that the White House could dial back its commitments. Stoltenberg said the significant increase was a sign that NATO allies were convincing the White House that they were sincere about increasing defense spending.

Leaders may also agree that NATO would formally join the global coalition fighting the Islamic State, a mostly symbolic step that would nevertheless give the alliance a permanent voice in coalition decisions. Tillerson said Wednesday that the step was a U.S. goal.

France is now the lone holdout, NATO officials said, because it is worried that counterterrorism is better addressed through policing and intelligence. The issue will be addressed at the lunch with Macron.

Despite Europe's overall nervousness about the visit, European officials and business leaders have said in recent weeks that they have grown somewhat more optimistic on at least a few fronts.

"It was a decision of the citizens of the United States to expressly not elect a president with political experience," said Jürgen Hardt, the foreign policy spokesman for Germany's ruling Christian Democratic Union and a close ally of Chancellor Angela Merkel. "The outcome is that U.S. policies are not clear at the moment," he said.

But he said that Merkel's visit in March had paved the way for smoother relations between Europe and the United States.

Faiola reported from Berlin. Karen DeYoung and Stefano Pitrelli in Rome and Stephanie Kirchner in Berlin contributed to this report.