The North Atlantic Treaty Organization is a military alliance that was created in 1949 in the aftermath of World War Two.
Its initial purpose was to deter the Soviet Union expansion in Europe.
But when the Cold War ended, the organization was reconceived as a "cooperative-security" alliance.
Currently, 30 countries from North America and Europe are members of NATO.
The alliance started off with only 12 founding membersand gradually expanded to include more over the years.
Most recently, Finland and Sweden have applied to join and are now in the accession process.
At the core of NATO's founding treaty is the principle of "collective defense" that brings members together to safeguard their freedom and security.
This idea of collective defense is enshrined in Article 5 of the North Atlantic Treaty.
It reads: "an attack against one shall be considered as an attack against all," committing them to protect and assist each other.
So far, Article 5 has been invoked once by the U.S., following the 9/11 terrorist attacks.
Today, in response to Russia's aggression against Ukraine, NATO is stepping up its collective defense measures.
It's deployed more than 40-thousand troops and transported significant air and naval assets to the eastern part of the Allianceto ensure the security of its members.
"NATO's solidarity was not very strong before the Ukraine war. But Russia's invasion of Ukraine spurred fears that such attacks could be made on any NATO member, prompting NATO's solidarityand triggering non-NATO members to join the alliance."
At this year's summit, NATO leaders are expected to thoroughly discuss how to respond to the new security challenges in Europe.
Min Suk-hyen, Arirang News.