As Russia’s invasion of Ukraine enters its second month, the role of the North Atlantic Treaty Organization has become evident.

Earlier Thursday, NATO leaders gathered for an emergency summit in Brussels and are expected to announce new sanctions against Russia as well as other measures to help bolster Ukraine’s defences. What they won’t do, however, is what Ukraine’s president has repeatedly demanded: impose a no-fly zone over Ukraine. US and NATO officials have repeatedly said such a move would risk provoking Russian President Vladimir Putin and triggering a wider war with Russia.

A discussion of the position of NATO forces along its eastern edge was also part of the last-minute diplomatic brilliance. And the leaders discussed what to do if Russia deploys a chemical, biological or even nuclear weapon, a prospect that is causing growing concern as the war reaches a stalemate. In a statement released afterwards, US President Joe Biden said NATO was “as strong and united as it has ever been”.

But what is NATO and who is part of it? NATO is a European and North American defense alliance created to promote peace and stability and to guarantee the security of its members. It was created as the Cold War escalated and is headquartered in Brussels, Belgium.

The goal of the US-led alliance 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.

Twelve founding countries – the United States, Canada, the United Kingdom, France and eight other European nations – signed the North Atlantic Treaty in 1949, pledging to protect each other by political and military means. .

Over the decades that followed, the alliance grew to include a total of 30 members.

In alphabetical order, these are:

Ukraine is not a member of NATO, but has long hoped to join the alliance. This is a sore point for Russia, which views NATO as a threat and vehemently opposes it.

NATO is currently led by Secretary General Jens Stoltenberg, whose term was extended for a year on Thursday. Stoltenberg strengthened NATO’s position in answer to the decision, saying, “As we face the greatest security crisis in a generation, we are united to keep our Alliance strong and our people safe.”

According to a Western official who was on hand for Biden’s remarks to leaders at the NATO summit, he referenced his US presidential predecessor when he pleaded for allies to increase defense spending. While it’s unclear whether Biden mentioned former President Donald Trump by name, Biden implored leaders to increase their defense budgets during this moment of crisis, but asked that they “not confuse him not” with his predecessor, who Biden said didn’t “treat NATO allies very well,” the official said. The White House declined to comment.

Learn more about NATO here.

CNN’s Natasha Bertrand contributed to this article.