Union of Lublin: Formation of the Polish–Lithuanian Commonwealth

By the mid-16th century, the political and military dynamics between Poland and Lithuania reached a tipping point. Since the Union of Krewo (1385), the two states had been linked by a personal union through shared monarchs, but Lithuania remained a distinct political entity. However, the increasing pressure from Muscovy’s expansion and Lithuania’s reliance on Polish military aid began to shift the balance of power. The Union of Lublin (1569) formalized this evolving relationship, transforming Poland and Lithuania into a single federal state, known as the Polish–Lithuanian Commonwealth.

Throughout the 15th and 16th centuries, Poland sought to integrate Lithuania fully into its political system, but Lithuanian nobles resisted, valuing their independence and patrimonial control over Ruthenian lands. Despite this, Lithuanian leaders grew more dependent on Polish financial and military support during conflicts with Moscow, especially after defeats in the Muscovite–Lithuanian Wars.

The pressure to create a more permanent union increased during the reign of Sigismund II Augustus, the last Jagiellonian king, who had no heir. With his death looming, Polish nobles feared that the personal union between Poland and Lithuania would collapse, leaving both countries vulnerable to external threats. At the same time, Lithuania’s reliance on Poland had become essential, especially after the Tatars and Moscow threatened Lithuanian territory.

In 1569, Sigismund summoned both Polish and Lithuanian leaders to negotiate the terms of a permanent union at Lublin. However, Lithuanian nobles were unhappy with the proposed land and property rights that would allow Polish nobles to acquire land in Lithuania. When they left in protest, Sigismund annexed key Ruthenian territories, including Volhynia and Kiev, into Poland. This move secured the support of Ruthenian nobles, who welcomed the greater privileges offered by Polish law.

Faced with this territorial loss and increasing pressure, the Lithuanian elite agreed to sign the Union of Lublin on July 1, 1569, though they remained wary of full integration. In return, they secured guarantees that Lithuania would retain some autonomy through separate institutions, such as its own army and state offices.

Poland and Lithuania after the Union of Lublin (1569). © Halibutt

The Union of Lublin officially established the Polish–Lithuanian Commonwealth, a joint polity ruled by an elected monarch. The two states would share a common foreign policy, currency, and Sejm (parliament), but retain distinct military and administrative systems. The Grand Duchy of Lithuania retained its title and institutions, though now subordinate to the Crown of Poland.

The death of Sigismund II Augustus in 1572 tested the union, as the two countries held joint royal elections for the first time. Lithuanian nobles remained cautious about Polish influence, even threatening to select a separate monarch if pressured. Despite these tensions, the Commonwealth emerged as a major power in Eastern Europe, capable of confronting Moscow and the Ottoman Empire.

The Union of Lublin marked a pivotal moment in Lithuanian history. It preserved the Grand Duchy’s sovereignty, but also confirmed Lithuania’s increasing cultural and political integration with Poland. This shift set the stage for future developments in the Commonwealth, including joint military campaigns, political reforms, and cultural exchanges. However, the arrangement also sowed seeds of future internal conflicts, as Lithuania’s elite struggled to balance autonomy with cooperation within the Commonwealth.