NATO — the North Atlantic Treaty Organization, to give it its full name — is a non-aggressive European and North American defense alliance set up to promote peace and stability and to safeguard the security of its members.The organization, which has its headquarters in Brussels, Belgium, was created as the Cold War escalated. Its aim was to protect Western European countries from the threat posed by the Soviet Union and to counter the spread of Communism after World War II.In April 1949, its 12 founders — the United States, Canada, the United Kingdom, France and eight other European nations — signed the North Atlantic Treaty, pledging to protect each other by political and military means.Over the decades since, the alliance has grown — today it has 30 members. In alphabetical order, they are: Albania, Belgium, Bulgaria, Canada, Croatia, Czech Republic, Denmark, Estonia, France, Germany, Greece, Hungary, Iceland, Italy, Latvia, Lithuania, Luxemburg, Montenegro, the Netherlands, North Macedonia, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Turkey, the U.K. and the U.S.Since the end of the Cold War, more than a dozen countries from the former Eastern bloc, including three former Soviet republics, joined the alliance. Russia still views NATO as a threat despite the collapse of the Soviet Union. Amid recent tensions with the West, Russia has asked for iron-clad guarantees that the alliance won't expand further — something NATO's members have resisted.Despite the major geopolitical changes since NATO's foundation, its purpose remains the same. The key principle underpinning the alliance is one of collective defense: "An armed attack against one or more of them in Europe or North America shall be considered an attack against them all."What does that mean in practice?The principle of collective defense is laid out in Article 5 of the North Atlantic Treaty. It guarantees that the resources of the whole alliance can be used to protect any single member nation. This is crucial for many of the smaller countries that would be defenseless without their allies. Iceland, for example, has no standing army.Since the U.S. is the largest and most powerful NATO member, any state in the alliance is effectively under U.S. protection.In reality, the first and only time Article 5 has been invoked was in the aftermath of the Sept. 11, 2001, attacks on the U.S.; as a result, NATO allies joined the invasion of Afghanistan.However, NATO has taken action on other occasions too. It put collective defense measures in place in 1991 when it deployed Patriot missiles during the Gulf War, in 2003 during the crisis in Iraq, and in 2012 in response to the situation in Syria, also with Patriot missiles.All three were based on requests from Turkey.Does it have its own army?No. NATO relies on its member countries contributing forces, meaning it is essentially as strong as the individual forces of each nation. It is in the interest of the entire coalition to make sure each country is putting enough resources into its defense.This has been one of the main sticking points in the alliance, with the U.S. and the U.K. often criticizing other member states for not putting in their fair share.American military spending has always eclipsed other allies' budgets since NATO's founding in 1949. But the gap grew much wider when the U.S. beefed up its spending after the 9/11 attacks.Video above: We 'don't know' if Russians are playing games, White House saysUnder the NATO guidelines, each country should be spending 2% of its GDP on defense, but most countries are not reaching that target.Former U.S. President Donald Trump was particularly vocal on this topic, demanding European countries do more and at one point even suggesting they "repay" the U.S. for their past shortfalls.According to the most recent estimates from NATO, seven member states, Greece, the U.S., Croatia, the U.K., Estonia, Latvia, Poland, Lithuania, Romania and France hit the 2% goal in 2021.Still, this is a significant improvement. In 2014, only the U.S., the U.K. and Greece were spending more than 2%.At that time, all member countries that were below the threshold committed to ramping up military spending to reach the target within a decade. Most are sticking with the promise.How has the organization's remit changed over time?After the collapse of the Soviet Union, NATO evolved and expanded.Over the years since, its members have served as peacekeepers in Bosnia, fought against human trafficking, and been deployed to intercept refugees in the Mediterranean.The alliance is also responding to the new ways conflicts can unfold, for example by setting up a cyber defense hub in Estonia.What might it do in Ukraine?NATO already had troops in eastern Europe before the most recent increase in tensions with Russia, but it has boosted its presence there in recent weeks.According to NATO, there are currently four multinational battalion-size battlegroups in Estonia, Latvia, Lithuania and Poland, on a rotational basis.These battle groups are led by the U.K., Canada, Germany and the U.S. NATO said on January 7 they are "robust and combat-ready forces."NATO Secretary General Jens Stoltenberg said the alliance is prepared to rapidly reinforce that presence further by sending additional forces and capabilities into the area.The Biden administration has put as many as 8,500 U.S. troops on heightened alert for a possible deployment to eastern Europe.More immediately, several NATO countries have started sending weapons and ammunition to Ukraine.Video above: US holds firm on Ukraine, awaits Russian responseThe U.S. has sent two shipments of weapons to Ukraine, including 300 Javelin anti-tank missiles, 800 bunker-busting bombs and hundreds of thousands of rounds of ammunition. The U.K. has supplied Ukraine with new light anti-tank weapons, and the Czech government agreed on Wednesday to donate more than 4,000 152-millimeter caliber artillery shells.Why is Germany being criticized?Berlin has come under criticism recently for its policy of not exporting weapons to crisis areas.Germany has so far refused to send arms to Ukraine, promising instead to give Kyiv a field hospital, medical training and 5,000 military helmets.Germany's complicated history means its governments have always been cautious about military spending and the idea of getting directly involved in a conflict is a difficult one to sell there. It has also been criticized for failing to reach the 2% GDP spending target, along with other countries."The German government has very clearly agreed that we will not send any lethal weapons, or arms deliveries to conflict areas because we do not want to fuel these conflicts further," German Defense Secretary Christine Lambrecht said Thursday.Germany hosts more than 30,000 U.S. military forces and their families.The U.K. came to Germany's defense on the issue on Thursday, with British Defense Secretary Ben Wallace saying his country doesn't judge Germany for its decision."The advantage of being in NATO is there are 30 allies, so we can all assist Ukraine in our own way," he said.''Obviously, the United Kingdom has taken a view that lethal aid of a tactical defensive nature is something that the Ukrainians need, but we're not sitting in judgement over other countries,'' Wallace said.