BRUSSELS (Reuters) - U.S. Defense Secretary Jim Mattis arrived in Brussels on Wednesday seeking to mollify European allies, again finding himself in the familiar position of mending relationships frayed by President Donald Trump’s policies.

U.S. Defense Secretary Jim Mattis testifies before a Senate Armed Services Committee hearing on the “Defense Department budget posture in review of the Defense Authorization Request for FY2019 and the Future Years Defense Program” on Capitol Hill in Washington, U.S., April 26, 2018. REUTERS/Aaron P. Bernstein

Trump infuriated European Union members, Canada and Mexico by imposing tariffs of 25 percent on steel and 10 percent on aluminum, which experts say could bleed into U.S. security relationships, even among America’s closest allies.

While the meeting of NATO defense ministers in Brussels is a routine affair, it will be watched closely in European capitals.

On his debut trip to Brussels as secretary of defense last year after Trump questioned the need for NATO, Mattis told members that while they must honor military spending pledges, the alliance was “the most successful and powerful military alliance in modern history.”

“This ministerial is going to be overshadowed by the tariffs issue,” said a U.S. defense official, speaking on the condition of anonymity.

“Mattis is going to be looking to reassure allies, try to strengthen relationships and continue to press the need for them to live up to their 2 percent commitment,” the official added.

The United States has been pushing NATO countries to reach a target to spend 2 percent of economic output on defense every year by 2024.

Speaking with reporters on the way to Brussels, Mattis suggested that while talks with NATO partners would be tough, he did not expect Trump’s tariffs decision to have an impact on defense issues for now.

“Along the path going there, certainly it’ll be a little rocky, a little bumpy at times, but so far I do not anticipate any effect in the security domain,” he said.

Barry Pavel, a U.S. national security expert at the Atlantic Council think-tank, said while issues of national security are usually unaffected by politics, Trump’s decision was far too provocative not to have an impact.

“I think it was such a shock and the reactions from several allies being so unprecedented and so emotional and insulted, that I think some of that will bleed into some of the discussions at the sort of geopolitical level,” Pavel said.

In response to Trump’s decision to impose tariffs, the EU threatened tariffs on Harley Davidson motorcycles and bourbon, measures aimed at the political bases of U.S. Republican legislators.

While in Brussels, Mattis will also meet his Turkish counterpart as Washington tries to balance Ankara’s concerns about Kurdish fighters among U.S.-backed SDF forces trying to retake the Islamic State’s remaining territory in Syria.

“How do we take Turkey’s legitimate security concerns and enhance their security... at the same time, the SDF was the only organization at the time that was able to throw ISIS off the track and defeat them in the field in very, very tough fighting and we will not simply cast that organization aside,” Mattis said.

Turkey and the United States on Monday endorsed a roadmap for the northern Syrian city of Manbij and underlined their mutual commitment to its implementation.

WANING INFLUENCE?

The informal portfolio of soothing traditional U.S. friends upset by Trump’s often sharp comments and tweets on foreign policy is one that the retired Marine general is becoming used to.

Less than a week ago in Asia, Mattis attempted to do much the same with allies Japan and South Korea, where some officials are concerned that Trump may put U.S. security interests ahead of theirs in pursuing a peace deal with North Korea.

In recent months, however, there have been growing questions about whether Mattis’ voice in internal administration debates could increasingly be drowned out by those of other advisers and the president himself, who increasingly trusts his own instincts.

In one notable instance, Mattis publicly suggested sticking with the Iran nuclear deal.

Dismayed European allies also sought to salvage the Iran nuclear deal and preserve their Iranian trade after Trump withdrew the United States from the landmark accord and ordered sanctions reimposed on Tehran.