Vladivostok, a thriving city situated at the end of the Trans-Siberian Railway, was once closed to foreigners and most Russians because of its strategic military importance. Its navy was based
there. It was opened by Mikhail Gorbachev near the end of the Cold War and during his period of
"Glasnost". Now people from different countries may visit Vladivostok freely.
Situated on Golden Horn Bay, which empties into the Sea of Japan and the Pacific Ocean, it is
still the headquarters of Russian Pacific naval operations.
According to the passage above, why was Vladivostok once closed to
foreign countries?