Why has the Trump administration focused heavily on burden sharing so far in its discussions with European leaders? Because for decades, Europe hasn’t spend enough on its own defense.

Following the end of the Cold War, European nations slashed defense spending in favor of domestic priorities. From 1992-1999, defense spending for European NATO members dropped 22%.

More than 25 years since the Berlin Wall came down, the issue of tepid defense spending among our European NATO allies remains a serious problem. American Presidents of both parties have raised the issue repeatedly with their European counterparts -- appeals that have largely been ignored.

What are NATO members expected to spend on defense? According to Article 3 of the North Atlantic Treaty which established NATO, member states are treaty obligated to “maintain and develop their individual and collective capacity to resist armed attack.”

Despite recent signs that some nations are beginning to wake from the continent’s collective security spending hibernation, it’s clear that many of our allies in Europe have failed to do so. In 2006, in an effort to encourage defense investment, NATO set a target for member states to spend 2% of GDP on defense. At the 2014 Wales Summit, member states again committed to spending 2% of GDP of defense and 20% of defense budgets on “major equipment” purchases.

In 2016, only five member states hit the 2% guideline, Estonia, Greece, Poland, the United Kingdom, and the U.S. Only ten member states spent the required 20% of their defense budgets on major equipment.

Today, a significant gap remains between what our allies our actually spending on defense and what they have committed to. A former colleague calculated that in 2015, the gap between the amount of money spent by NATO countries and the amount that would have been spent should each have spent 2% of their GDP on defense amounted to a $100 billion gap.

That number is truly significant and reason enough for the U.S. to continue to advocate for our NATO partners to spend more on defense. At the same time, it is also important to acknowledge that recently defense spending has been increasing, and to acknowledge those nations that are working to live up to their spending commitments.

An underreported story is that in the past two years, changing threat perceptions among our allies has led to increases in defense spending in some countries. In 2015, 15 NATO members increased defense spending in real terms; in 2016, 16 NATO allies raised defense spending as a share of GDP. Put another way, in 2016, collectively the other non U.S. members of NATO increased spending 3.8% or $10 billion.

Nations which border or are near Russia have unsurprisingly taken the lead in defense increases. While the Baltic nation of Estonia has spent 2% of GDP on defense for years, Latvia and Lithuania will reach the 2% requirement next year. Romania will spend 2% of GDP on defense this year. Norway, which shares an Arctic border with Russia is increasing defense spending by 4.2% in 2017. Poland plans to spend $15.6 billion on military modernization across the next five years.

Germany, a country which has received much press for its lack of defense spending has increased its defense investment by €2 billion in 2017, although even with the increase, Germany will only spend a paltry 1.22% of GDP overall. French Chief of Defense Gen. Pierre de Villiers recently called for a €2.8 billion defense spending increase in 2018, and argued his country needs to spend 2% of GDP by 2022 rather than 2025. In December, Croatia, one of the newest NATO members approved its first defense spending increase in six years.

This is not to say all is well on European defense spending -- in fact, quite the opposite. Too many of our allies are shirking their treaty obligations to maintain their capacity to resist armed attack. Weak defense spending by European NATO members threatens to undermine the collective security guarantee and play into Putin’s hands.

Therefore, it is right and proper for U.S. officials to encourage increased defense spending in Europe. Policymakers, however, should never forgot that remaining engaged in Europe serves America’s interests.

This spring, it is good to take a moment and notice the budding signs that some NATO allies at least may be beginning to stir from their long defense spending hibernation.