NATO isn't the only alliance that countries are eager to join – a brief history of the Five Eyes

The July 2023 NATO summit in Lithuania saw movement toward expanding the alliance. Paul Ellis/Pool Photo via AP

After the recent NATO summit in Vilnius, Lithuania, it is anticipated that Sweden will soon become the alliance’s 32nd member.

The heart of this alliance – which was established in the aftermath of World War II to promote the collective security of its mostly Western European members – is Article 5 of the North Atlantic Treaty, which requires that if one member is attacked, then all of the other members will respond as if they themselves had been attacked.