What You Need to Know About NATO Amid Trump's Skepticism

Donald Trump raised doubts about whether the United States, under his leadership, would jump to the defense of its NATO allies in Europe if Russia attacked them.

When specifically asked in an interview with The New York Times Wednesday about his views of Russia, Trump said that if it attacked some of the small Baltic states, which are the most recent members of NATO, he would decide whether to come to their aid only after reviewing whether those nations "have fulfilled their obligations to us."

NATO's collective defense agreement requires all member countries to come to the aid of any member state that is attacked.

NATO Secretary General Jens Stoltenberg responded to Trump's remarks with a call for unity. "Solidarity among allies is a key value for NATO," he told Buzzfeed. "This is good for European security and good for U.S. security. We defend one another."

So what exactly is NATO? ABC News breaks down the organization's history, importance and criticisms below:

What Is NATO?

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ATO stands for the North Atlantic Treaty Organization, a security alliance established in 1949 during the early days of the Cold War to counter Soviet aggression in Europe.

Now numbering 28 countries in Europe and North America, the alliance's goal is to "safeguard the freedom and security of its members through political and military means," NATO's website reads.

The organization promotes "democratic values" and encourages member nations to work together on issues of defense and security to prevent long-term conflict.

When security disputes occur, NATO advocates peaceful resolutions. But there are guidelines for how military force can be used, outlined in Article 5 of the Washington Treaty, the founding treaty of NATO.

NATO adheres to a policy of collective defense, meaning an attack on one member is considered "an attack against all." The policy is outlined in Article 5 and has only been invoked once, after the Twin Towers in New York City were attacked Sept. 11, 2001, and NATO members sent troops to Afghanistan.

After the Taliban fell, a United Nations Security Council resolution established the International Security Assistance Force (ISAF), under NATO's control, to stabilize the country. There were 1,044 non-U.S. NATO service members killed fighting in Afghanistan.

How Does NATO Work?

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eadquartered in Brussels, Belgium, each member nation is represented by an ambassador that sits on the North Atlantic Council (NAC), the alliance's political decision-making body. The NAC meets at least once a week and is chaired by Secretary General Stoltenberg, the former prime minister of Norway.

When political decisions require the military, NATO's Military Committee is involved in the planning and resourcing of military elements needed for an operation. While NATO has few permanent military forces, member nations can voluntarily contribute forces when the need arises.

The Military Committee is made up of the Chiefs of Defense of NATO-member countries; the International Military Staff, the Military Committee's executive body; and the military command structure, composed of Allied Command Operations and Allied Command Transformation.

Where Is NATO Operating Right Now?

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urrently, NATO's website lists five active operations and missions: Afghanistan, Kosovo, counter-piracy off of East Africa, monitoring the Mediterranean, and supporting the African Union.

Who Pays for NATO?

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ATO recommends that member countries spend 2 percent of their gross domestic product (GDP) on defense.

Currently, according to the Wall Street Journal, only five members meet that goal: the United States, Great Britain, Greece, Estonia, and Poland.

Latvia and Lithuania are two Baltic states that don't meet the target, but those countries are likely to raise their defense spending in the face of growing Russian aggression.

The United States spent the most on defense in 2014, over 3.5 percent of the GDP. That accounts for approximately 75 percent of the military spending by all NATO members, according to the WSJ.

What Is the History Behind Its Origin?

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he North Atlantic Treaty was signed April 4, 1949, in the aftermath of World War II and rising geopolitical tension with the Soviet Union.

NATO's website lists three purposes for its creation: "deterring Soviet expansionism, forbidding the revival of nationalist militarism in Europe through a strong North American presence on the continent, and encouraging European political integration."

As the Cold War settled in, NATO stood in opposition to the Soviet bloc, communist nations that allied with the Soviet Union.

In 1991, after the Soviet Union dissolved, NATO developed partnerships with former adversaries.

NATO responded to its first major crisis response operation in 1995 in Bosnia and Herzegovina after the Bosnian Civil War.

More recently, NATO responded to the Libyan crisis in 2011 by carrying out airstrikes to protect civilians under attack by the Gaddafi regime.

Is Trump Alone in His Criticism of NATO?

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o. Trump isn't the first to criticize other NATO members for contributing less than the United States

In 2011, Defense Secretary Robert Gates called the future of NATO "dim" if other nations didn't increase their participation in allied activities.

"The blunt reality is that there will be dwindling appetite and patience in the U.S. Congress -- and in the American body politic writ large -- to expend increasingly precious funds on behalf of nations that are apparently unwilling to devote the necessary resources or make the necessary changes to be serious and capable partners in their own defense," he said.

It should be noted that Gates made these comments prior to Russia's annexation of Crimea in 2014 and escalating regional tension there.

Trump's criticisms of NATO are not only that member nations aren't contributing fairly, but also that the organization itself is no longer relevant.

"I think NATO is obsolete," Trump told ABC News in March. "NATO was done at a time you had the Soviet Union, which was obviously larger -- much larger than Russia is today."

NATO's history is fraught with waves of criticism, often in moments of relative peace. After the fall of the Soviet Union, critics alleged that a European alliance was no longer necessary to counter communist governments. But militant nationalism was still occurring and soon NATO was put to the test with the Balkan Wars. Indeed, changing security threats have consistently pushed NATO to evolve over the past 60 years.

But NATO's website perhaps provides the best defense of itself:

"Since its founding in 1949, the transatlantic Alliance's flexibility, embedded in its original Treaty, has allowed it to suit the different requirements of different times. In the 1950s, the Alliance was a purely defensive organization. In the 1960s, NATO became a political instrument for détente. In the 1990s, the Alliance was a tool for the stabilization of Eastern Europe and Central Asia through the incorporation of new Partners and Allies. Now NATO has a new mission: extending peace through the strategic projection of security.

"This is not a mission of choice, but of necessity. The Allies neither invented nor desired it. Events themselves have forced this mission upon them. Nation-state failure and violent extremism may well be the defining threats of the first half of the 21st century. Only a vigorously coordinated international response can address them. This is our common challenge. As the foundation stone of transatlantic peace, NATO must be ready to meet it."

ABC News' Brian McBride contributed to this report.