What is NATO, and why isn’t Ukraine a member?

Military trucks carry British tanks at Estonia's NATO battle group base in Tapa, Estonia, on Feb. 25. (Sergei Stepanov/AP)
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In the weeks leading up to Russia’s invasion of Ukraine, the North Atlantic Treaty Organization, a military alliance of mainly Western countries, deployed thousands of additional troops to Eastern Europe to shore up its defenses.

Since then, member countries have delivered weapons, ammunition and other military aid to Ukrainian forces.

NATO ministers discuss boosting Ukraine — and avoiding a wider war

But Ukraine does not belong to NATO, whose 30 members are united by a mutual defense treaty. And post-Cold War tensions between the West and Russia over the alliance’s eastward expansion — including to countries that were once part of the Soviet Union — are at the heart of the current crisis.

Ukraine has made clear its desire to join NATO, even rewriting the constitution to enshrine its commitment. Russia, however, sees NATO enlargement as an existential threat and has demanded that Ukraine be barred from ever becoming a member.

Russian President Vladimir Putin has called the expansion of NATO a “red line” for him, and has cited it as a reason for invading Ukraine.

Though Ukraine has backed away from seeking immediate membership into the alliance, two other countries — Finland and Sweden, both traditionally nonaligned — have been inching closer to joining NATO.

On Thursday, Finland’s leaders announced that they would seek NATO membership as soon as possible — an extraordinary move that would double the alliance’s land border with Russia. The Kremlin immediately hit back, saying Finland’s accession to NATO would “definitely” threaten Russia’s security.

“The expansion of NATO does not make our continent more stable and secure,” Kremlin spokesman Dmitry Peskov told journalists Thursday, according to Russian news outlet Interfax. He added that Russia could take new measures to “balance the situation” if Finland joins the alliance.

Here is some essential background about NATO and how nations are able to join.

How joining NATO and the E.U. became Ukraine’s unattainable dream