Let's first look at the geographical location of Ukraine, the east of Ukraine is Russia, the north is Belarus, the west is all NATO members except for the small country of Moldova, and the south is the Black Sea.
If Ukraine joins NATO, Russia will lose both ukraine's military buffer belt and sea access to the Black Sea and the Mediterranean Sea.
Military buffer belt
At present, NATO's most advanced military bases on the northern shore of the Black Sea are Romania and Poland, and if Ukraine joins NATO, then NATO's most advanced military bases can be placed on the Russian border, the nearest location is less than 500 kilometers from Moscow, within the firing range of short-range missiles.
Tanks and other land kings can also reach the Russian border directly through Poland, which is a great threat to the defense of Russian territory.
Black Sea and Mediterranean Sea Access
Russia has a Black Sea Fleet in the Black Sea, one of the four major Naval fleets of Russia, with several bases in the Black Sea and the Sea of Azov, the main of which is in the Crimean port city of Sevastopol. Since the collapse of the Soviet Union, the Black Sea Fleet has been leased from Ukraine ever since. Crimea held a referendum in 2014 and was incorporated into the Russian Federation.
In 2018, Donetsk and Luhansk became independent, and on February 21, 2022, Russia announced the recognition of the independence of Donetsk and Luhansk. Russia thus took control of the Sea of Azov.
For the Black Sea Fleet to go to sea, it needs to enter the Mediterranean Sea through the Turkish Strait, which is controlled by Turkey.
The Montreux Convention signed by the Black Sea states in 1936 severely restricted the duration and tonnage of non-Black Sea warships entering and leaving the Black Sea, while the Black Sea states were free to pass through the Turkish Strait in peacetime except aircraft carriers, and recognized Turkey's sovereignty over the strait, which Turkey had the right to blockade in time of war or neutrality.
In 1991, the Soviet Union collapsed, Russia inherited the Black Sea Fleet in Ukraine Crimea in the form of a lease treaty, the Black Sea Fleet mobility was greatly weakened, with the collapse of the Warsaw Pact and NATO's eastward expansion, the Russian Black Sea Fleet completely lost control of the Black Sea, and NATO warships also entered and exited the Black Sea many times.
In 2014, Crimea entered Russia and the Black Sea Fleet regained its life. Since the outbreak of the Syrian civil war, the port of Tartus in Syria, as Russia's only overseas base outside the CIS, has become a military material support base for Russia's assistance to Syria, with the continuation of the Syrian war and Russia's deep involvement, tartus naval base will become a strategic fulcrum of the Russian Navy, which will help alleviate the dilemma of the Black Sea Fleet.
To date, the Turkish Strait has long been the throat of the Black Sea countries, and the Montreux Convention remains the international legal basis for Black Sea navigation.
If Ukraine joins NATO, the Black Sea Fleet will lose its foothold in the Black Sea, and Russia's influence in the Middle East and Africa will be greatly reduced.