A major eastern European polity allied with Poland at Grunwald.
The Grand Duchy of Lithuania was medieval Europe's great anomaly: the last pagan state, forged by Mindaugas (crowned 1253) and expanded by Gediminas and Algirdas until it ruled more Orthodox Rus' than pagan Balts — from the Baltic toward the Black Sea, Kiev itself taken after Blue Waters (1362).
Squeezed between the Teutonic Order's perpetual crusade and Moscow's rise, it chose the west: Jogaila's baptism and Polish crown (1386), Christianisation from 1387, Vytautas's great-power decades — the Battle of Grunwald (1410) shared, the steppe frontier at the Vorskla lost (1399) — and the long constitutional dance with Poland that led, past the period's edge, to full union.
The Baltic tribes of the Nemunas basin — never Christianised, never conquered by Rus' — consolidated under pressure: the sword-brothers' and Teutonic Order's crusades from the 1230s made unity survival. Mindaugas united the duchies, accepted baptism for a royal crown (1253, Christendom's newest king), then apostatised or was ambiguous, and was assassinated (1263): the experiment's end, but not the state's.
The pagan grand dukes who followed ran Europe's strangest chancery: Gediminas (1316–1341) — Vilnius's founder-by-letter, inviting German merchants and friars while rebuffing baptism, marrying his children across Orthodox and Catholic courts — building by diplomacy an empire his sons Algirdas and Kęstutis ran as a remarkable diarchy: Algirdas east against Moscow (thrice at its walls), Kęstutis west against the Order's annual reisen (the crusading package-tours of European chivalry — Bolingbroke among the tourists).
Dynastic struggle after Algirdas — Jogaila against uncle Kęstutis (strangled at Kreva, 1382) and cousin Vytautas — resolved into the union of Krewo (1385): Jogaila baptised, married to Jadwiga of Poland, crowned Władysław II Jagiełło; Lithuania's official conversion following from 1387 (Samogitia, the Order's coveted corridor, only after 1413). The Order's reason-for-being dissolved just as its power peaked.
Vytautas, reconciled as grand duke (Astrava 1392, effectively sovereign), made the duchy a great power: Tatar politics reaching for the steppe (the catastrophic Vorskla, 1399, checking the Black Sea dream), Smolensk taken (1404), Grunwald's co-command (1410) — the union's armies destroying the Order — the Horodło union (1413) adopting Lithuanian boyars into Polish heraldic clans, and the Congress of Lutsk (1429) offering him a crown he died awaiting (1430): the duchy's zenith in one lifetime.
The duchy was a federation under the Gediminid kin: patrimonial grand-ducal core (Vilnius, Trakai) ringed by Rus' principalities ruled by dynasts or local princes under charters — "we do not destroy the old, we do not impose the new", the formula of Ruthenian autonomy. Chancery Slavonic (Ruthenian) served as state language beside Latin and German diplomacy: a pagan, then Catholic, elite governing an Orthodox majority in their own tongue.
After Horodło (1413) Catholic boyars gained Polish-style rights and offices (voivodes, castellans of Vilnius and Trakai), the council of lords (Rada) hardening into the real government during absentee reigns; privileges of 1432–1447 extended rights across confessional lines to stem Muscovite attraction. The Statutes (1529, past the period) would codify what medieval practice built: Europe's largest state run as legal federation.
Mindaugas, unifier, king 1253–1263 — the crowned experiment and its assassination.
Gediminas, 1316–1341 — Vilnius, the letters to Europe, the dynastic web: the empire's architect.
Algirdas, 1345–1377 — Blue Waters (1362), Kiev, and thrice to Moscow's walls.
Kęstutis, co-ruler to 1382 — the west's defender through the Order's crusade-decades.
Jogaila (Władysław II Jagiełło), 1377–1434 — Krewo's convert: Lithuania's duke as Poland's king.
Vytautas the Great, 1392–1430 — Grunwald, Lutsk, and the duchy's zenith.
Two-front war defined it: against the Teutonic Order, a century of reisen, castles (Kaunas taken and retaken), Samogitian risings, and the decision at Grunwald (1410) with Melno's peace (1422) finally fixing the border; against and across the steppe, Blue Waters (1362) taking the Kiev lands from the Horde, Vorskla (1399) burying Vytautas's Tatar grand design, and the long duel with Moscow beginning as Algirdas's sieges and continuing as attrition over the Orthodox borderlands.
Expansion was mostly absorption: Polotsk, Vitebsk, Volhynia, Kiev, Smolensk — Rus' principalities entering the Gediminid federation by conquest, marriage, and charter, until the duchy stretched Baltic-to-Black-Sea steppe: the largest state in Europe governed from wooden Vilnius.