Strains in the U.S.-European relationship are peaking just as President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE prepares to face allies at this month’s critical NATO summit.

Trump has repeatedly complained that members aren’t meeting the alliance’s defense spending goal. But the meeting comes at a difficult time, with the broader U.S.-European relationship already under stress from Trump’s tariffs, his withdrawal from the Iran nuclear deal and his overtures to improve relations with Russian President Vladimir Putin.

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Those looming issues have raised fears of a replay of last month's contentious Group of Seven summit. The mood of the tense summit was embodied in a photo released by German Chancellor Angela Merkel's office, which showed her staring down a cross-armed Trump.

“We’ll have a repeat of the G-7 with Trump going to the summit and likely complaining once again about allies not spending enough money on defense," said James Goldgeier, a professor of international relations at American University. "And doing it in a public way designed to basically heighten the notion that he seems to have that NATO is just another bad deal for the U.S.”

Since his presidential campaign, Trump has railed against NATO allies for what he sees as an unfair reliance on the United States to foot the bill for their defense.

Just eight of NATO's 29 members currently meet or are expected this year to meet the alliance’s goal of spending 2 percent of their gross domestic product on defense: the United States, United Kingdom, Greece, Poland, Latvia, Estonia, Romania and Lithuania.

That goal was set at the 2014 Wales summit, where allies agreed to meet the target by 2024. NATO’s secretary-general has said at least 15 allies will make the 2024 deadline.

Trump, though, wants allies to speed up their spending plans. He upped the ante recently in pointed letters to Merkel, Canadian Prime Minister Justin Trudeau and leaders in Belgium and Norway, according to excerpts published Monday by The New York Times.

In the June letters, Trump hinted he may be considering a shift in U.S. military posture should the leaders not increase their defense spending.

“It will, however, become increasingly difficult to justify to American citizens why some countries do not share NATO’s collective security burden while American soldiers continue to sacrifice their lives overseas or come home gravely wounded,” Trump wrote to Merkel, according to the Times.

The Pentagon is conducting a cost-benefit analysis of the 35,000 troops deployed to Germany, as first reported Friday by The Washington Post. A Pentagon spokesman insisted to reporters Monday the review is routine and not in anticipation of a White House demand to withdraw troops.

The personal acrimony between Trump and Merkel, in particular, has appeared to grow in recent months, with the president tweeting recently that “the people of Germany are turning against their leadership.”

At the G-7, Trump reportedly tossed two pieces of Starburst candy to Merkel and said, “Here, Angela. Don’t say I never give you anything.”

Trump also rattled allies at the G-7 by saying Russia should be allowed back into the group. Russia was kicked out after annexing Crimea in 2014 in a move Western nations maintain was illegal.

Trump also reportedly disparaged NATO during the G-7 summit saying the alliance is “as bad as NAFTA,” the North American trade agreement he wants to renegotiate or rip up.

The increased tensions come after Trump went ahead with 25 percent tariffs on steel and 10 percent tariffs on aluminum from the European Union, Canada and Mexico. All have announced retaliatory tariffs.

Trump also bucked European allies when he decided to withdraw from the Iran deal, causing them to scramble to find ways to protect their businesses from U.S. sanctions in an effort to save the agreement.

The divisions prompted NATO Secretary-General Jens Stoltenberg to warn in a recent op-ed in The Guardian that “nowhere is it written in stone that the transatlantic bond will always thrive.”

“That doesn’t, however, mean that its breakdown is inevitable,” he continued. “We can maintain it, and all the mutual benefits we derive from it.”

Trump’s first meeting with NATO leaders last summer was rocky. He declined to endorse the mutual defense provision known as Article 5, though he ended up doing so a month later under questions from the press. A video clip of Trump pushing aside the leader of NATO’s newest member, Montenegro, also went viral.

Expectations for this year’s summit are not high.

“I’m expecting it to be tense, but it’s in everyone’s interest that we get through this without any major incidents,” said Luke Coffey, director of the Allison Center for Foreign Policy at the conservative Heritage Foundation.

But despite the public animosity between Trump and European leaders, several experts said the day-to-day operation of the alliance has gone on smoothly.

U.S. troops continue to rotate through Eastern Europe and conduct exercises with their counterparts from Poland, the Baltic states and other NATO countries.

Congress is also on track to increase spending for the European Deterrence Initiative to $6.5 billion next year, in line with the Trump administration’s request. The fund was created in 2015 to reassure allies nervous about a resurgent Russia.

“Certainly at the head of the state level, it’s fraught with tension, but with an undercurrent of pragmatism, I would say, behind the scenes,” Coffey said of U.S.-European relations. “The relationship is healthier at the institutional level, so between foreign ministry and State Department, or ministry of defense and Department of Defense, or military-to-military.”

Still, some worry change is coming during next week’s trip to Europe.

During Trump’s summit with North Korean leader Kim Jong Un, he agreed to cancel joint U.S.-South Korean military exercises, calling them provocative and a waste of money.

NATO supporters are fearful of Trump doing the same to European military exercises at his summit with Russian President Vladimir Putin, which is scheduled days after the NATO summit.

“If he does something similar to what he did in Singapore, then that would be a market change,” Goldgeier said, referring to the Korea summit.

With the NATO and Putin summits scheduled back-to-back, a group of leading Senate Democrats wrote to Trump urging him to remember who is a U.S. ally and who is an adversary.

The Monday letter was penned by Senate Foreign Relations Committee ranking member Bob Menendez Robert (Bob) MenendezKasie Hunt to host lead-in show for MSNBC's 'Morning Joe' Senators ask for removal of tariffs on EU food, wine, spirits: report VOA visa decision could hobble Venezuela coverage MORE (D-N.J.), Senate Minority Whip Dick Durbin Richard (Dick) Joseph DurbinFeinstein 'surprised and taken aback' by suggestion she's not up for Supreme Court fight Grand jury charges no officers in Breonna Taylor death Democrats shoot down talk of expanding Supreme Court MORE (D-Ill.), Senate Intelligence Committee Vice Chairman Mark Warner Mark Robert WarnerDemocrats call for declassifying election threats after briefing by Trump officials It's time to upgrade benefits Intelligence chief says Congress will get some in-person election security briefings MORE (D-Va.) and Senate Armed Services Committee ranking member Jack Reed John (Jack) Francis ReedWhen 'Buy American' and common sense collide Hillicon Valley: Russia 'amplifying' concerns around mail-in voting to undermine election | Facebook and Twitter take steps to limit Trump remarks on voting | Facebook to block political ads ahead of election Top Democrats press Trump to sanction Russian individuals over 2020 election interference efforts MORE (D-R.I.).

“In addition to urging NATO allies to meet their commitments to spend 2 percent of GDP on defense, working to boost NATO rapid mobility and readiness capacities, and addressing cyber threats and other evolving forms of hybrid warfare,” the senators wrote, “it is imperative that you make a strong statement of support for the democratic nations that make up the alliance and make clear that the United States stands with—not in opposition to—our oldest and closest allies.”

The letter highlighted the apprehension ahead of Trump's next summit.

Coffey, though, struck one hopeful note.

“I think expectations at this point are so low that we’ll probably come out the other end of the summit and say, ‘Oh, it wasn’t so bad after all,’ ” he said.