Moldova is uniquely located at the frontier of Eastern and Western cultures. This has contributed to a long and difficult struggle by the indigenous Moldovan people to maintain their cultural and political sovereignty. During the Middle Ages (as in modern times) Moldova, like other principalities in the region, engaged in efforts toward the maintenance and recognition of its independence, the defense of its territory, and the preservation of its borders. One of the country's most glorious eras occurred during the reign of Stefan cel Mare (Steven the Great) between 1457 and 1504. During these years, Moldovans won impressive victories over the Turks, Tartars, Hungarians, Poles, and other invaders. This temporary success, however, was no guarantee of the nation's future stability. Under the permanent threat of invasion, the principalities of this region unified as a means to resist aggression. The first unification-of Transylvania, Moldova, and Muntenia-took place during the rule of Mihai Viteazul (1593 to 1601). Although short-lived, this event served as a precedent for the union of the principalities of Tara Romaneasca and Moldova to form a new country, Romania, in 1859. Following the Crimean War, political stability in the area was fleeting. In 1856, Russia lost the southern region of Basarabia to Moldova, only to gain it back from Romania in 1878 at the Congress of Berlin. With the collapse of the Russian Empire in 1918, this area, made up of part of the present territory of Moldova and part of Ukraine, declared its independence and reunited with Romania. The newly formed Union of Soviet Socialist Republics refused to recognize this reunification, however, and in 1924 created the Moldavian Autonomous Soviet Socialist Republic. In June 1940, after Hitler and Stalin signed the 1939 Molotov-Ribbentrop pact of nonaggression, the Soviet Union annexed additional territory to form the Moldavian Soviet Socialist Republic (MSSR). In the early 1940s, as World War II raged in Europe, Romania again claimed the territory of the MSSR. As the war came to a conclusion, the Soviet Union annexed the region again for a final time. Subsequently, in an attempt to create a uniform culture among the Soviet republics, the leadership of the Soviet Union began a period of intense Russification, replacing Moldovans' traditional Latin alphabet with the Cyrillic alphabet and Romanian with Russian as the official language. The tone of Soviet leadership changed in 1986 with the introduction of a policy of glasnost (openness) by President Mikhail Gorbachev. This new policy permitted the pursuit of traditional culture by the Moldovan population and leadership and set the stage for the republic's independence. On August 27, 1991, Moldova declared its independence, an event that is now celebrated every year on that date. To restore the cultural heritage of the majority of its citizens, the Moldovan government reestablished Romanian, using the Latin alphabet, as the national language.