Turkey is probably the most "Essential" member of NATO because of the Turkish Straits.
The two narrow straits in northwestern Turkey, the Bosphorus and the Dardanelles, that connect the Sea of Marmara with the Black Sea on one side and the Aegean arm of the Mediterranean Sea on the other.
https://en.wikipedia.org/wiki/Turkish_Straits
The Montreux Convention Regarding the Regime of the Straits is a 1936 agreement that gives Turkey control over the Bosporus Straits and the Dardanelles and regulates the transit of naval warships.
Articles 20 and 21 of the 1936 Montreux Convention:
Article 20 - In time of war, Turkey being belligerent, the provisions of Articles 10 to 18 shall not be applicable; the passage of warships shall be left entirely to the discretion of the Turkish government.
Article 21 - Should Turkey consider herself to be threatened with imminent danger of war she shall have the right to apply the provisions of Article 20 of the present Convention.
If Turkey invokes either article, Russia is legally blocked from moving any military ship through the Straits. That is unless the Russians attack the Straits, and force a transit by military force, a clear act of war against Turkey.
https://en.wikipedia.org/wiki/Montreux_Convention_Regarding_the_Regime_of_the_Straits
Calm