The desire of Ukraine to become part of NATO, a Western military alliance led by the United States, is among the top reasons for the ongoing war in the Eastern Europe. Ukraine becoming part of NATO poses an existential threat to Russia.
NATO (North Atlantic Treaty Organisation)

• NATO was set up in 1949 by the US, Canada and several western European nations to ensure their collective security against the Soviet Union.

• It was the first peacetime military alliance by the US outside the western hemisphere.

• Member countries: it is an intergovernmental political and military alliance of 30 countries, among which there are 28 Europeans and two North American countries.

• Headquarters:It is headquartered in Brussels, Belgium. The headquarters of Allied Command Operations is near Mons, also in Belgium.

• Political alliance: NATO promotes democratic values and enables members to consult and cooperate on defence and security related issues.

• Military alliance: NATO is committed to peaceful resolution of disputes and it has the military power to undertake crisis management operations if diplomatic efforts fail.

• Mutual defence:Members of NATO are committed to mutual defence in response to an attack by any external party.

• Collective defence: collective defence lies at the very heart of NATO,a unique and enduring principle that binds its members together, committing them to protect each other and setting a spirit of solidarity within the Alliance.

• It is laid out in Article 5 of the North Atlantic Treaty, the founding treaty of NATO.

According to Article 5, the parties agree that an armed attack against one or more of them in Europe or North America shall be considered as an attack against all of them. They only agree that is such an armed attack occurs each one of them in exercise of the right of individual or collective self-defence as recognised by Article 51 of the charter of United Nations would assist the party under attack, by such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area.

Genesis

• Rebuilding economy after WWII: The battered European nations started rebuilding their economies after the end of World War II.

• Preventing westward expansion of USSR:It was at this time that the US believed that an economically strong, re-armed and integrated Europe was critical for preventing the westward expansion of Communist USSR.

• Thus, the  US embarked on a programme to supply economic aid to the continent on a massive scale.

• Marshall plan: The Marshall plan (named after President Harry S Truman’s Secretary of State George C Marshall), i.e the European Recovery Programme promoted the idea of shared interests and cooperation between the US and Europe.

• Response of USSR towards Marshall plan: The USSR declined participation in the Marshall plan and discouraged Eastern European states in its fear of influence from receiving economic assistance from America.

• Greek Civil War: The US and UK worked to thwart the Soviet backed Communist takeover of Greece in 1946-49 Greek Civil War.

• The western nations threw their weight behind Turkey as it stood up to Soviet pressure over control of Bosporus(connecting the Black Sea & Sea of Marmara) and the Dardanelles Strait (which connects the Sea of Marmara & Aegean Sea).

• The US committed itself to containing the Communist uprisings in Greece and Turkey in 1947-48.

• Coup in erstwhile Czechoslovakia: The government of Stalin sponsored a group in erstwhile Czechoslovakia in 1948 which led to installation of a Communist regime in a country sharing border with both Soviet controlled East Germany and the pro-West West Germany.

• In 1948-49, the Soviets blockaded West Berlin to force the US, UK, and France to give up their post-war jurisdictions in the country, leading to a major crisis.

• American-European alliance: All the events led to a conclusion that an American-European alliance was necessary against the USSR.

• The Europeans are also convinced of the need for a collective security solution.

• Brussels Treaty of collective defence: The UK, France, Belgium, Netherlands and Luxembourg signed the treaty in 1948 according to which if any of the signatories faced attack, they would be defended by all the others.

• Vandenburg Resolution: The US Congress passed the landmark resolution after the Brussels treaty advising the US President to seek US and free world security through support of mutual defence agreements.

• It was the stepping stone to NATO, the US believed the treaty would be more effective, if it included more signatories from the countries of the north Atlantic.

• Establishment of NATO: The treaty was signed in Washington DC in 1949, it initially had 12 signatories: The US, UK, Canada, France, Denmark, Belgium, Norway, Portugal, the Netherlands, Italy, Iceland and Luxembourg.

TENSIONS WITH RUSSIA

• The reason for the establishment of NATO was the hostility to the USSR.

• Warsaw Pact:The Soviet Union signed its own collective defence treaty called the Warsaw Pact with seven Eastern European countries: Poland, Czechoslovakia, Albania, Bulgaria, East Germany, Hungary and Romania.

• Collapse of Warsaw Pact and expansionism of NATO: The Warsaw Pact collapsed with the end of the Cold War and was formally declared disbanded in 1991.

• The USSR, Czechoslovakia and East Germany no longer exist and the remaining five of its signatories are now part of NATO.

• Concerns of Russia: Russia has been suspicious and insecure about the West.

• The three Baltic states (Estonia, Latvia and Lithuania) which are now part of NATO share borders with Russia.

• Belarus and Ukraine are the only countries which were once in Russia’s sphere of influence and are now outside the western military alliance.

• For Russia, keeping a buffer between NATO and Russia along its southern and western border border is critical for its security.

• A hostile Ukraine protected by NATO‘s nuclear umbrella could potentially have missile launch pads within a few hundred kilometres of Moscow and can cut off access of Russia to the warm water ports of Black Sea; it was in part to pre-empt this eventuality that Russia annexed Crimea in 2014.