NATO heads of state will gather Thursday in Brussels for a so-called mini-summit (officially a meeting of the North Atlantic Council at the heads-of-state level). But if the alliance wants to attain real progress on increasing European defense spending, then NATO finance ministers should also make the trip to Belgium.

The mini-summit will be the first meeting of NATO heads of state since President Donald Trump took office. Not surprisingly, burden sharing, or rather the lack thereof, will feature high on the agenda.

In 2016, the United States accounted for over 68 percent of NATO defense expenditures, while accounting for only around 46 percent of the gross domestic product, or GDP, of member nations.

Trump isn’t alone in his concern about the disproportionate U.S. share of defense spending and capabilities within NATO. Presidents of both parties have sought to persuade our allies to spend more on defense, with little to show for it.

Leaving the U.S. aside, last year only four other members of the 28-member alliance dedicated 2 percent of GDP to defense, and only nine spent the required 20 percent of defense expenditures on equipment.

James Jay Carafano and Andras Simonyi recently wrote for The Hill that the essential question underpinning low defense spending is a lack of political resolve among NATO allies to prioritize such spending over other priorities.

Germany is the obvious example. In 2016, that country posted a budget surplus of over $26 billion, its highest surplus since the fall of the Berlin Wall. Yet, this year, Germany’s defense spending will increase only by the relatively small amount of $2.2 billion, raising its overall defense investment level to 1.22 percent of GDP in 2017.

Our German allies, unfortunately, are hardly the worst offenders. The Netherlands, one of the wealthiest countries in Europe, spent only 1.16 percent of GDP on defense in 2016. Italy spent even less at 1.11 percent, and our Spanish allies dedicated a measly 0.90 percent of GDP to defense.

A continued lack of investment among most of our allies is why a new approach is needed, one which involves ministers of finance. As Carafano and Simonyi explain:

Sitting next to their country’s leaders, in the company of the foreign and defense ministers, the finance ministers’ very presence would signal a full understanding of the importance of defense. Far from being ‘a waste,’ defense spending is an essential element underpinning our free and democratic societies, upon which economic prosperity depends.

American policymakers sometimes fail to appreciate the power that finance ministries have in European governments. Finance ministers and defense ministers are often at loggerheads over the need for more defense investment.

Including finance ministers at the NATO table could help forge common ground. European heads of state could enlist them as partners in explaining to the public why defense spending needs to be prioritized.

A relatively peaceful, stable Europe has led to economic, cultural, and military dividends that are far greater than the amounts the U.S. spends on military personnel and bases on the Continent.

In Brussels, Trump should state clearly and unequivocally that it is in America’s national interest to remain actively engaged in Europe and a leader inside NATO.

At the same, the president has every right to raise the issue of inadequate defense spending among our allies.

As an intergovernmental security alliance, NATO is only as strong as its member states. While some allies have begun to reverse the trend of declining defense spending, more must be done.

Trump should use Thursday’s meeting to press allies on defense spending in a responsible and realistic way. Including finance ministers in these important discussions would increase the likelihood of real results down the road.