By the early 13th century, the unification of the Baltic tribes gained momentum, driven by external threats from the Livonian Order and Teutonic Knights. The shift from fragmented tribes to a centralized state began with the 1219 treaty, when 21 Lithuanian dukes, including a young Mindaugas, signed a peace agreement with Galicia–Volhynia. This marked the first documented sign of Lithuanian consolidation, though tribal leaders still held significant autonomy. The threat from German religious orders further encouraged unification, especially after the Livonian Order’s defeat at the Battle of Saule in 1236.
Mindaugas emerged as the most powerful leader among the Lithuanian dukes. By the 1240s, he had consolidated power through alliances, military campaigns, and strategic marriages. However, his rise involved internal conflicts, including wars against his nephews and other rival dukes. Mindaugas converted to Christianity in 1251 and was crowned King of Lithuania in 1253, temporarily establishing Lithuania as a Christian kingdom and securing papal recognition to stabilize his rule.
Despite this, Mindaugas' support for Christianity wavered. Encouraged by his nephew Treniota, he broke peace with the Livonian Order, likely abandoning his Christian faith. Mindaugas focused on expanding Lithuania eastward, seizing lands weakened by the collapse of Kievan Rus'. However, growing tensions between Mindaugas and his allies culminated in his assassination in 1263, leading Lithuania into a period of instability.
In the years following Mindaugas' death, leadership changed hands frequently, with seven grand dukes ruling over the next 32 years. Despite internal strife, Lithuania did not fragment. By 1295, Vytenis assumed power, laying the groundwork for future expansion. The Grand Duchy of Lithuania, having withstood threats from both internal divisions and external enemies, was now positioned to grow into a powerful and enduring state.