NATO headquarters in Brussels, Belgium (James K. McCann/U.S. Army)

When President Donald Trump arrives at NATO headquarters on Thursday, allies will be under intense pressure to live up to demands from an American administration that has threatened to make the thorny issue of increased allied defense spending a condition for guaranteeing U.S. commitment to 68-year-old military pact.

During the one-day meeting of 28 heads of state, the first such gathering since the election of NATO-skeptic Trump, members are likely to hear demands for concrete plans for boosting investment in their respective militaries. Trump is also expected to seek greater commitments from allies for the ongoing mission in Afghanistan as well as a larger NATO role in the battle against the Islamic State group in Iraq and Syria.

The question is whether allies can summon the will to meet such demands, which are a source of political tension in many of Europe’s capitals, or push back against Trump’s tough talk and risk a rift with the U.S. administration.

“He (Trump) is at a place where he would like to stay in NATO, but if there isn’t progress much quicker, then he would rather not stay in NATO,” a senior Trump administration official told reporters last week. “We’ll either see real changes towards NATO or we’ll try to form a different way of going about things.”

The election of the unconventional Trump has caused anxiety among Europe’s political establishment, which has struggled to make sense of his conflicting views. Yet Trump’s harsh words about NATO’s relevance and mixed signals about American commitment to the alliance could also spur needed reform, some experts say.

“The reality — counterintuitive as it may sound — is that the Trump administration could be the catalyst for long overdue changes for the alliance,” wrote former NATO officials Fabrice Pothier and Alexander Vershbow in a new Atlantic Council report issued Tuesday.

Since Dwight D. Eisenhower was president, virtually every U.S. administration has complained about NATO members’ underinvesting in defense while enjoying the protection that comes with U.S. treaty obligations to safeguard the Continent. Despite the complaints, the disparity in spending has only increased.

During the Cold War, the U.S. accounted for 50 percent of defense expenditures among allies, compared with today’s 68 percent, according to the Atlantic Council report dubbed “NATO and Trump, the case for a new Transatlantic Bargain.” That relative increase is due to the downsizing of Europe’s armed forces after the collapse of the Soviet Union.

In the past two years, the trend has begun to reverse, with many NATO states increasing defense investment in response to a more assertive Russia. In 2016, defense spending increased by $10 billion, or 4 percent, across the alliance.

Still, the Atlantic Council report argues that allies should pledge to close at least half the defense spending gap by 2020, with all members hitting the target of investing 2 percent of gross domestic product in their respective militaries by 2024.

Leaders of some NATO states, however, argue that too much attention has been paid to the issue of defense spending and the alliance’s 2 percent benchmark. Some critics have noted that the benchmark, which first surfaced in the late 1990s, was an arbitrary figure picked for political reasons and not directly linked to capabilities, readiness, or other specific military requirements.

In late March, Secretary of State Rex Tillerson delivered a two-month deadline for NATO states to craft plans for boosting defense spending to agreed levels. Germany balked at the demand, setting up a potential conflict between Berlin and Washington at NATO’s conference.

Instead, the focus should be on how capable militaries are and whether members are willing to take part in operations in places such as Afghanistan, some members argue. Germany also wants expenditures such as development aid to be counted as investment in security.

But those arguments run up against a Trump administration that has emphasized military dollars and cents.

“Trump and his administration are not going to be satisfied with good intentions and encouraging trends and statistics,” wrote Versbow and Pothier. “What is needed to address U.S. concerns and guarantee that European and Canadian defense budgets stay on an upward trajectory is an interim plan — over enough years to spread defense investment, but not so many that concrete decisions can be pushed off to a distant future.”

Thursday’s meeting in Brussels could see allies taking a first step toward drawing up individual national plans.

If NATO is to boost its overall capabilities, a key will be getting larger members to increase spending, not just smaller eastern European states that have taken steps to reach NATO targets. While Germany, Spain, Italy, the Netherlands, and Canada represent about 25 percent of NATO’s GDP, those countries spending adds up to just 10 percent of total NATO defense expenditures.

Besides seeking a greater financial commitment from allies, Trump wants NATO to step up its support for operations against the Islamic State group. NATO has a small training mission in Iraq, but it could expand those efforts and increase intelligence and surveillance operations in the region.

NATO Secretary-General Jens Stoltenberg has acknowledged that the alliance can do more to support the U.S.-led coalition. Allies are also working on plans to add troops to NATO’s mission in Afghanistan, where U.S. military officials have said more troops are needed.

Details are likely to be unveiled during talks in Brussels.

Last week, Gen. Curtis M. Scaparrotti, NATO’s top military commander, said he expected the issue of increased troop contributions to be dealt with when heads of state meet.

While NATO shifted from a war fighting mission in 2014 to primarily one of advising and training the Afghan military, there appears to be no end in sight to the military campaign, now in its 16th year.

“If we think we can leave, we will find we are back because we are fighting a global terrorist, we’re fighting global terrorists today and that will be another place where they will use to launch attacks, so we can succeed and I think we must succeed,” Scaparrotti told reporters during a recent meeting of defense chiefs.

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