CARL AZUZ, cnn 10 ANCHOR: It's been 70 years since NATO, the North Atlantic Treaty Organization, was formed. The world's changed a lot since then. The NATO alliance has grown a lot since then. And yesterday's visit by its leader to the U.S. capital is our first subject on today's show.
Even though the United States was a founding member of NATO in 1949, Wednesday was the first time that a NATO secretary general ever addressed the U.S. House and Senate.
During a speech that promoted the organization itself and the unity of its members, Jens Stoltenberg acknowledged that NATO countries have their disagreements. But he said they've overcome them in the past, and they'll have to do it again, because the challenges that NATO faces can't be addressed by one country alone.
JENS STOLTENBERG, NATO SECRETARY GENERAL: Questions are being asked on both sides of Atlantic about the strength of our partnership. And yes, there are differences. This is democracy. Open discussions and different views is not a sign of weakness. It is a sign of strength.
NIC ROBERTSON, cnn INTERNATIONAL DIPLOMATIC EDITOR: What is NATO? Why is it important and what's its future?
The North Atlantic Treaty Organization is a political and military alliance established in 1949 that seeks to promote stability in the North Atlantic area.
HARRY TRUMAN, FORMER U.S. PRESIDENT: It is the will of the people of the world for our freedom and for our peace.
NIC ROBERTSON: Led my Secretary General Jens Stoltenberg, there are 29 member countries, and its HQ is in Brussels. NATO doesn't have its own troops, but relies on contributions of forces from its member countries.
At NATO's core is Article V, which states an attack on one member is an attack on all NATO allies.
The collective defense principle was to protect Western European nations against the Soviet Union. But when the Soviet Union collapsed, NATO's new tasks ranged from being a bulwark against Al-Qaeda in Afghanistan to fighting human trafficking and intercepting refuges in the Mediterranean.
NATO is still extremely active, with some 4,000 U.S. troops in Poland and the Baltic states, and tens of thousands on 48-hour standby, bolstering NATO's allies and sending a clear message to Russia.
AZUZ: Secretary General Stoltenberg's speech on Wednesday came a day after he met with U.S. President Donald Trump at the White House. And Stoltenberg referred to the American leader in his speech. President Trump has been skeptical of NATO in the past.
The defense alliance recommends its members spend at least 2 percent of their gross domestic product on defense. Only seven out of 29 actually do that. The U.S. does and Stoltenberg credits President Trump with getting other members to spend more.
Despite that and other international disagreements over its policies, NATO has lasted longer than any other defense alliance in recorded history, and several international analysts say it's the most successful alliance too.