Snake Island, also known as Serpent Island or Zmiinyi Island, is an island located in the Black Sea off the coast of (what used to be) Ukraine, near the Danube Delta, with an important role in delimiting what (used to be) Ukrainian territorial waters. It is now Russian and Russia means to keep it.
The island has been known since classical antiquity, and during that era hosted a Greek temple to Achilles.
In the Special Military Operation that Russia began in February to de-militarize and de-Nazify Ukraine, the island became the focus of storied battles; with Ukraine claiming their soldiers bravely faced-off against Russian naval vessels, which allegedly killed all the Ukrainian defenders.
Of course, that story proved false, when Russia released unharmed, all the soldiers from Ukraine they had taken prisoner when Russia conquered the island.
During the ongoing hostilities between Ukraine and Russia, several high profile attempts were made by Ukraine to retake the island. All failed.
This week, Russia deployed its famous S-400 air defense system on the island. The S-400 is widely known as the most accurate and deadliest air defense system in the world.
Looks as though Russia means to keep what it has, and the S-400 seems perfectly able to make sure it does.