Article 5 of the North Atlantic Treaty states that “an armed attack against one” member of the treaty “shall be considered an attack against them all.” This principle of collective deterrence is the cornerstone of the North Atlantic Treaty Organization (NATO) and is a major reason why the United States and our allies in Europe have enjoyed over 60 years of peace and prosperity since the end of World War II.

Just as important, but less well known, is Article 3 of the Treaty, which obligates all NATO members to “maintain and develop their individual and collective capacity to resist armed attack.”

In 2006, NATO members agreed to fulfill this commitment by pledging to spend at least two percent of their gross domestic product (GDP) on defense spending every year. Ten years later, less than half of NATO members have honored that pledge.

As a result, the United States has taken on a disproportionate amount of the defense related burden compared to what was originally contemplated in the North Atlantic Treaty. According to NATO’s own figures, even though the GDP of the United States is smaller than the combined GDP of all other NATO member countries, the United States contributes an astonishing 73 percent of all NATO member defense spending.

This does not mean the U.S. is paying 73 percent of all NATO related costs, like running the headquarters in Brussels. But it does mean that NATO and its member nations are overly dependent on U.S. intelligence, surveillance, reconnaissance, ballistic missiles, and air-to-air assets.

In fact, while some NATO members like Poland are pulling their own weight (Poland spends about 2.18 percent of GDP on defense), 14 NATO member nations spend less on defense than the city of New York spends on its police department.

This pattern of shirking shared responsibilities not only departs from the original intent of the North Atlantic Treaty, it also jeopardizes the peace and security that the alliance has helped maintain for more than 60 years. Today, NATO member nations face a host of new security challenges – from an increasingly antagonistic Russia to a burgeoning immigration crisis (largely of their own making). In order to meet these challenges, they must increase their defense spending.

That’s why Sen. Jeff Sessions (R-AL) and I sent a letter to President Obama this week, asking him to make increased NATO defense spending a top priority at next month’s NATO Summit in Warsaw, Poland.

“It is simply inconceivable to most Americans,” the letter reads, “that their hard-earned tax dollars are used to reassure financially capable allies who have failed to meet decade-old commitments.”

Ultimately, the elected officials of NATO member countries, and the people they represent, must choose to change course and once again invest in the defensive resources that keep us safe. But as the commander in chief, President Obama has a unique responsibility to encourage our allies to reaffirm their commitment to the collective security of Europe and the United States.