What is NATO and why is President Trump slamming it?

WASHINGTON — President Donald Trump’s broadsides against NATO have rattled a 69-year military alliance that largely defined the global order following World War II.

But Trump isn’t the only modern U.S. president to raise questions about the reliance of European nations on American defense spending.

Here's a glimpse of NATO's history, its mission and its funding sources.

NATO's historic roots

The North Atlantic Treaty Organization was created in 1949 with Europe still reeling from the war's devastation. With an eye toward communist expansion aided by the Soviet Union, the U.S., Canada and 10 other nations decided a system of collective defense was in every members’ best interest.

A centerpiece of the treaty, which was signed by then President Harry Truman, is the Article 5 provision that requires member states to come to the aid of their allies in the event of an attack.

The provision has been invoked only once - in 2001 when the U.S. was the target of terrorist attacks.

Global military might

NATO has since grown to 29 member states that, according to the organization’s website, are committed to “guarantee the freedom and security of its members through political and military means.”

The organization is headquartered in Brussels, and summits are generally held every year or two.

Historians have credited NATO with unifying Western democracies during the Cold War, but the organization has faced internal struggles. France formally pulled out of the group’s military command in the 1960s, angering the U.S., and then rejoined in 2009.

Who pays for NATO?

Trump, who ran on a platform of putting “America First,” has a long history of questioning U.S. involvement overseas. He has specifically railed against NATO members for not contributing more money to their own defense.

He’s not the first to raise such questions. President Barack Obama, speaking at a press conference shortly after the Russian annexation of Crimea in 2014, said he was concerned “about a diminished level of defense spending among some of our partners in NATO.”

Months later, NATO members pledged to spend at least 2 percent of their gross domestic product on defense by 2024. So far, only five countries are meeting that goal — the U.S, the United Kingdom, Greece, Estonia and Latvia. Others have noted they still have several more years to meet the target.

The U.S. spends the highest share of GDP, at 3.5 percent, and Luxembourg spends the lowest, at just over half of 1 percent.