Russian President Vladimir Putin has repeatedly pointed to NATO's post-Cold War enlargement as one of the many reasons for his invasion of Ukraine. But Russia's war has ultimately pushed the alliance on a path to further expansion.
The latest: Formally submitting applications last month, Finland and Sweden are poised to soon join NATO. Their future accession became even more likely on Tuesday after Turkey lifted its objection to the two Nordic countries joining the alliance.
The big picture: Finland and Sweden's membership would dramatically change the security landscape in Europe and more than double the length of the alliance's borders with Russia.
Below is a brief look at why NATO formed, why Finland and Sweden are seeking membership, and where the alliance's likely expansion leaves Russia and Ukraine.
Finland and Sweden have for decades been NATO's closest partners, despite their official "non-alignment."
Finland's President Sauli Niinistö says his country is seeking NATO membership because Russia's invasion proved that the Kremlin does not respect officially non-aligned countries.
Sweden's Prime Minister Magdalena Andersson also pointed to Russia's actions.
NATO Secretary-General Jens Stoltenberg previously said the two Nordic countries would be quickly "welcomed with open arms."
Ukraine, along with Georgia, sought to join NATO at the Bucharest Summit in 2008, viewing it as a route to independence from Russian influence, per the Washington Post.
Ukraine's hopes of joining NATO were then dashed in 2010 when Viktor Yanukovych was elected president. After taking office, Yanukovych called joining the alliance an "unrealistic prospect."
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Editor's note: This story has been updated with new details from the NATO Summit in Madrid.