Jadwiga, crowned "king" of Poland in her own right in 1384, accepted marriage to the pagan grand duke Jogaila to Christianise Lithuania — the union that created the Jagiellonian commonwealth and, at Grunwald, broke the Teutonic Order.
Jadwiga was crowned at Kraków in October 1384 not as queen but as king — rex Poloniae — the Polish lords' declaration that sovereignty itself sat in the ten-year-old girl. Two years later she accepted the price of a policy grander than her own betrothal-hearted preferences: marriage to Jogaila of Lithuania, pagan Europe's last great ruler, baptised as Władysław on the eve of the wedding.
The union of Krewo turned two rivals into a dual power the length of Eastern Europe, brought the last pagan nation into Christendom by treaty rather than Teutonic sword — destroying the Order's whole reason for existing, a generation before Grunwald destroyed its army — and made Jadwiga, in her decade of co-rule, the union's conscience: diplomat to the Order, patron of the restored Kraków university, saint of its memory.
Born at Buda in 1374, youngest daughter of Louis the Great — king of Hungary and, from 1370, of Poland — Jadwiga was raised in one of Europe's most cultivated courts and betrothed in childhood to William of Habsburg, with whom she was partly brought up in Vienna.
Louis's death in 1382 broke the personal union: Hungary took her sister Mary, and the Polish lords, after two years' hard bargaining with her mother, took Jadwiga — on their terms. She arrived at thirteen and was crowned king on 16 October 1384, the male title securing her sovereignty against any husband's claims.
The contemporary and near-contemporary testimony — chancery records, Długosz's history a generation later, the canonisation depositions — describes intelligence and piety fused with unusual political discipline: a girl who reportedly took an axe to the locked castle gate to reach her Habsburg betrothed, then, persuaded of the union's Christian stakes, renounced him and never looked back. Her Latin, French, German, and Polish, her psalter-translations and library, made her court a bridge between Angevin sophistication and Polish need.
The recorded acts have a consistent grain: interceding for Kraków's townsfolk against her own retinue's damages ("who will give them back their tears?"), negotiating personally with the Teutonic grand masters where her husband's presence would have meant war, selling her jewels to refound the university that became the Jagiellonian. Poland's memory made her a saint; the documents make her something rarer — a teenage sovereign who chose policy over inclination once, completely, and governed by that choice for life.
The union of Krewo (August 1385) traded her hand for Lithuania's baptism: Jogaila undertook to convert himself and his nation, reclaim Poland's lost lands, and bind his grand duchy to the Polish crown. Baptised Władysław, he married Jadwiga in February 1386 and was crowned co-king; she remained crowned king in her own right, and the charters ran in both names.
Her decade of co-rule had its own portfolio: the 1387 expedition she led in person that recovered Red Ruthenia (Lwów's homage) from Hungarian occupation; the standing diplomacy with the Teutonic Order over Dobrzyń and Samogitia, where the grand masters preferred the queen's table to the king's camp; and the cultural foundation — the bequest of her jewels that reopened the University of Kraków in 1400, six months after her death, with theology faculty and Lithuanian scholarships aimed squarely at staffing the new Christian east.
Her only child, Elizabeth Bonifacia, was born on 22 June 1399 and died within three weeks; Jadwiga followed on 17 July, twenty-five years old. On her deathbed, the tradition runs, she counselled Jogaila to marry Anna of Cilli, granddaughter of Casimir the Great — handing her husband the dynastic legitimacy her own death took with it.
The structures she died for held: the union survived her by four centuries, Grunwald came eleven years later, and the Jagiellonian university still teaches under her name. Canonised by John Paul II in 1997 at Kraków, she remains the only woman crowned king of Poland — and the union's founding act remains, by any measure, one of the most consequential marriages in European history.