Turning 75 on Thursday (4 April), NATO will celebrate the success of its open-door policy that has charmed many over the decades but will remain keenly aware that it is facing possibly the biggest security challenges since its inception.

NATO started as a Cold War vehicle with 12 members in 1949, as a Western military alliance aiming to form a block against the Soviet Union.

Over the years, as the Soviet and then Russian threat diminished, NATO shifted its focus to the rise of terrorism, its southern flank, and the ways that China is challenging its security.

In addition to finding solutions to these issues, members also had to face tough internal questions.

The latest divisions over how to handle Ukraine’s accession gave some food for thought.

The same goes for questioning the existence of the alliance itself, with Donald Trump’s undermining of the mutual defence clause and French President Emmanuel Macron calling NATO “brain-dead”, as well as internal conflicts between France, Turkey, and Greece, and the difficult withdrawal from Afghanistan after the Taliban takeover of Kabul.

New memberships have stalled over the last decade, and it took Russia’s full-scale invasion of Ukraine in 2022 to pique the interest in a new wave of enlargement, resulting in the admission of Sweden and Finland.

The alliance’s members faced difficulties when Turkey delayed Sweden’s accession for almost two years and heavy pressure was put on Ankara by almost all governments.

The cracked unity was reinforced when Ukraine surprised them with an application just a few months into the war, leaving the 30 member countries in limbo and wondering how to best answer such a demand.

The decision to delay this to a later stage reminded all hopeful members of how difficult it is to join the club.

Further enlargement will depend on geopolitics, internal politics and reality on the ground.

Waves of enlargement

In 1952, to avoid one country blocking the other’s accession to the alliance, Turkey and Greece, which had in the past fought over territorial disputes, became members simultaneously.

Western Germany and Spain joined between the 1950s and the end of the 1980s, followed by East Germany in 1990 after its reunification with Germany.

After the fall of the Berlin Wall and the collapse of the Soviet Union, many ex-Soviet states joined, including Czechia, Hungary, and Poland in 1999.

The next wave of ex-Soviet accession came in 2004 with Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia, and Slovenia. This was followed by the slightly more sporadic accession of Albania and Croatia in 2009, Montenegro in 2017, and North Macedonia in 2020.

The enlargement has prompted Moscow to accuse NATO of breaking an alleged promise that it will not expand eastwards.

Russia’s attack on Ukraine in the winter of 2022 brought the focus back on Moscow, triggering new applications from Sweden and Finland, seeking the protection of Article 5, the mutual defence clause.

Kyiv made its membership request, while Kosovo also signalled its wish to join.

Military Alliance vs International politics

Even though it is not advertised as such, the membership process is heavily politicised, and dominated by the geopolitical priorities of its members. Turkey, for instance, requested that Sweden change its law to show evidence it took counter-terrorism seriously.

The latest wave of enlargement tightened NATO’s grip on the Baltic Sea, now dubbed the ‘NATO lake’, doubling its border with Russia and limiting Moscow’s access to the sea.

The issue of Ukraine’s membership has caused ire in some countries, which believe it is too early for such conversations. Germany and the United States remain cautious about too close a rapprochement with Ukraine, fearing it could drag the alliance directly into war.

The next wave of enlargement in Eastern Europe, possibly including Ukraine and Georgia, would bring NATO yet another step closer to Russia by erasing the buffer countries currently between the two blocs.

Next members?

Ukraine is most likely to be next, as it is currently waiting for an invitation. Security commitment packages signed bilaterally between Western nations and Ukraine will last ten years — the estimated time needed for Kyiv to reach the expected standards.

Countries such as the United Kingdom, France, Germany, Sweden, and Finland already signed agreements as promises for long-term deliveries of military, financial and humanitarian assistance to Kyiv. The overriding issue, however, is how long the war will last and what will be the outcome.

Georgia, which was promised membership in 2008, remains in the waiting room, with NATO officials pointing to problematic Russian influence and a lack of political reforms.

In the Balkans, Bosnia and Herzegovina is also following the Membership Action Plan toward accession, but the secessionist rhetoric of its Serbs and internal political tensions are putting obstacles in the reform processes.

Kosovo also wants to join but as four NATO members do not recognise its independence, declared from Serbia in 2008, this could be a problem.

Meanwhile, Serbia, an EU candidate, has no intention of applying due to its close relations with Russia, and also because of lingering grievances following the NATO bombing campaign in 1999 that halted Belgrade’s crackdown on Kosovo.

But for NATO diplomats talking to Euractiv, the enlargement process will not stop here as countries want to benefit from the alliance’s ability to bring peace and deter attacks.

A long accession processes

However, the path to NATO can be long for prospective members.

According to the Washington founding treaty of NATO, any European country that can contribute to guaranteeing the alliance’s collective security can join.

For an application to be considered, the country must be “invited” to become a candidate. This can take several years, for example, Ukraine has been waiting for two years.

Even though it is not clearly stated as such in the Treaty, NATO members consider that the only eligible countries are those whose governments respect the rule of law, where civilians control the military and are in control of their entire territory (i.e. are not at war).

Once invited, each member’s parliament and government or president must sign off on the accession, which, can take as little as a few hours.

Each country then sends the signed accession protocol to the US State Department and, once this is complete, the country becomes a full member of NATO.

Read more with Euractiv