The word “Lithuania” was mentioned for the first time in the annals of Quedlinburg (1009) where a story describes how Saint Bruno was killed by the pagans “on the border of Lithuania and Russ”.
It was the paganism that made Lithuania known in Western Europe in this era as the Christian kings were keen to convert this area to Christianity. The “Baltic Crusades” started and Teutonic Order HQ was moved to Marienburg not far from the Baltic Sea.
In this era, the small duchies of Lithuania consolidated into a larger country with its ruler Mindaugas adopting Catholicism and getting crowned in the year 1253. The change in religious policy was short-lived, however, and the next monarch Treniota went back to the defense of paganism and seemingly eternal war against the Teutonic Order that transformed Western Lithuania into depopulated wastelands.
While ruled by pagan Lithuanians the Grand Duchy successfully expanded eastwards and annexed many Orthodox lands in modern-day Belarus and Ukraine, therefore becoming the last pagan great power in Europe (and, in fact, the final pagan state).
In 1385 the monarch Jogaila converted his country to Catholicism and adopted Polish crown in addition to the Lithuanian one by signing the Union of Krėva. Grand Duchy of Lithuania still remained independent and reached its greatest extent under the rule of Vytautas the Great when it was the largest country in Europe stretching from the Baltic Sea to the Black Sea. By this time Orthodox Slavs outnumbered Catholic Lithuanians, but the latter remained in power.
It was a spectacular victory by Vytautas and Jogaila at Žalgiris (Grunewald) in 1410 against the Teutonic knights that finally defeated the Order as it had previously refused to drop territorial claims against Lithuania even after its conversion. Since that moment it was the rapidly expanding Russia (Muscovy) that posed the greatest threat to Lithuania capturing various cities of the Grand Duchy in a series of wars. Russian monarchs claimed that all of the Lithuania's Slavic lands should belong to Russia.
See also: Top 10 Medieval sites in Lithuania