In anticipation of this week’s summit meeting of NATO heads of state in Wales, and rising tension between the West and Russia, here’s a quick overview of what you need to know about the organization.

What is NATO?

The North Atlantic Treaty Organization is an alliance between the governments and militaries of 28 member countries in North America and Europe.

NATO was formed as a result of the 1949 North Atlantic Treaty, which was signed in Washington, D.C. You can watch the signing here.

The group originally consisted of 12 countries, including Canada, the United States, the United Kingdom and France. Over the subsequent 60 years, 16 additional countries have been added to the alliance, including Germany, Spain, and most recently, Albania and Croatia.

A complete list of member countries can be found here.

One of the main benefits of being a NATO member is the principle of “collective defence,” where “an attack against one or several members is considered as an attack against all.”

Essentially, if any NATO country were to be attacked, the other states would be obligated to respond as if the attack were against their own country. Article 5, which outlines this principle, has only been invoked once, in response to the 9/11 attacks in New York City.

Who can join?

NATO says that membership is open to “any other European state in a position to further the principles of this Treaty and to contribute to the security of the North Atlantic area.”

Outside of the 28 nations already a part of the organization, others can join the “Membership Action Plan,” which is a sort of preliminary stage to joining the alliance. Currently, the former Yugoslav Republic of Macedonia and Montenegro are part of the program. Nine countries have previously gone through this process to become full members.

NATO also has a network of “partners” and countries that co-operate with the alliance. A list of states associated with NATO can be found here.

Why was NATO formed?

Like today, NATO’s purpose has always seemed to be to oppose Russia. This was never clearer than in 1955 when the European continent almost completely divided into two camps: NATO on one side and the opposing Warsaw Pact -- which had a similar goal to NATO and a unified army under Soviet Rule – on the other. East and West Germany were split down the middle as the countries east of Austria and Yugloslavia formed an alliance of their own.

The countries in the Warsaw Pact viewed the remilitarization of West Germany – a new NATO member – as a threat to peace.

According to NATO, Russian opposition wasn’t the sole purpose for the group’s creation.

“In fact, the Alliance’s creation was part of a broader effort to serve three purposes,” reads their website. “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.

When has NATO been in action?

Following the fall of the Berlin wall and the collapse of the Soviet Union, NATO remained in action. The group’s first major crisis-management role was in Bosnia-Herzegovina in 1995, where a force oversaw security following the Bosnian war.

Following the September 11 attacks in 2001, NATO became involved in Afghanistan, and has also taken part in missions in Kosovo, Iraq and Libya.

And though it’s unlikely NATO will go to war over Ukraine, the country has recently expressed interest in becoming a NATO member.

Here is an interactive map showing current NATO missions. The map also has the option to highlight current organization members, the different types of NATO partners, countries that contribute troops as well as pop-up videos and stories.