During the Cold War, the Soviet Union, warned by the Walker spy ring — active from 1967 to 1985 — about the vulnerability of its nuclear submarines, concentrated them in the Barents Sea close to the Russian mainland. Later, Moscow did the same on the Pacific coast in the Sea of Okhotsk. With their most precious assets huddled in isolated waters, the Soviets then implemented what became known as the “bastion concept” to protect them. As the nuclear submarines maneuvered within a defined space, they were protected by approximately 75 percent of the Soviet navy's attack submarines, every surface vessel in its northern and Pacific fleets, and hundreds of aircraft. It was a truly formidable defense, one that NATO spent considerable energy and resources finding counters to.
Well, bastions are back. Worse, they are proliferating and are more formidable than ever.