Thirty-one countries from the alliance gathered for Exercise Trident Juncture last year (Picture: NATO).
NATO is marking 20 years since former Soviet countries began to join the alliance.
In that time, 13 countries have been welcomed into NATO, and enlargement could continue with more countries currently being considered.
When NATO was founded, it was created to defend against places like Hungary and Kosovo.
They were part of a swathe of communist Eastern European states heavily controlled by Moscow.
Hungary was a founder member of the Warsaw pact - Russia's eastern military alliance in answer to NATO in the West.
For 36 years they were locked in Cold War, but in 1991 the Warsaw pact dissolved with the fall of the Soviet Union.
Eight years later, Poland, Hungary, and the Czech Republic were admitted to NATO, starting a seemingly unstoppable march of the alliance’s border towards Russia.
In 2002 another seven countries joined, including three of Russia’s neighbours: Estonia, Latvia, and Lithuania.
NATO says it is open to discussing joining with any country – a policy that has almost doubled the number of countries in the alliance in just two decades.
Many of NATO’s newest members have been keen to contribute.
Estonians who served alongside British troops in Southern Afghanistan may have been relatively small in number, but compared to Estonia’s population it was one of the bigger contributors to the International Security Assistance Force (ISAF).
Amid ongoing tensions about underspending by many NATO members, four of the seven nations to actually meet the 2% target are former Warsaw pact countries.
NATO shows no sign of ending its expansion.
Russian neighbour Ukraine is one of three nations currently in discussions about membership.
It was Russia’s annexation of Crimea from Ukraine that renewed NATO’s focus on deterring Russia.
However, it has also caused some to question whether NATO’s expansion has provoked Russia and risked a new Cold War.