Several elements have interacted to influence the course and pattern of the boundaries and the regime of Soviet territorial waters. The foremost of these is national security. All of the seas bordering the U.S.S.R. have narrow entrances which can be commanded easily by hostile foreign Powers. During the Russian Revolution and subsequent Civil War, German vessels and, after the War, Allied vessels in the Baltic and the Dardanelles restricted to an uncomfortable extent the freedom of action of the Soviet Government. Soviet weakness in the Baltic theater was a major factor in determining Soviet policy towards Finland and the Baltic states during the 1939-1941 period, and the proximity of NATO naval forces to the Baltic continues to provoke Soviet proposals to close the sea to noncoastal Powers. Similarly, the U.S.S.R. was compelled to endure Turkish violations of the Montreux Convention on the Turkish Straits during World War II while its Black Sea fleet was immobilized. The Pacific coast seas and the Atlantic and Pacific approaches to the Arctic seas are also susceptible to a blockade by hostile Powers. Even the Arctic seas themselves, once regarded as an unguarded but impregnable frozen boundary, have become unexpectedly vulnerable with the development of nuclear submarines.