Polish–Lithuanian Commonwealth in the context of "Central Europe"

A political union is a type of political entity which is composed of, or created from, smaller polities or the process which achieves this. These smaller polities are usually called federated states and federal territories in a federal government; they are called prefectures, regions, or provinces in the case of a centralised government. This form of government may be created through voluntary and mutual cession and is described as unionism by its constituent members and proponents. In other cases, it may arise from political unification, characterised by coercion and conquest. The unification of separate states which, in the past, had together constituted a single entity is known as reunification. Unlike a personal union or real union, the individual constituent entities may have devolution of powers but are subordinate to a central government or coordinated in some sort of organization. In a federalised system, the constituent entities usually have internal autonomy, for example in the setup of police departments, and share power with the federal government, for whom external sovereignty, military forces, and foreign affairs are usually reserved. The union is recognised internationally as a single political entity. A political union may also be called a legislative union or state union.A union may be effected in many forms, broadly categorized as:

• Incorporating union
• Incorporating annexation
• Federal union
• Federative annexation
• Mixed unions

Ukraine is a country in Eastern Europe. It is the second-largest country in Europe after Russia, which borders it to the east and northeast. Ukraine also borders Belarus to the north; Poland and Slovakia to the west; Hungary, Romania and Moldova to the southwest; and the Black Sea and the Sea of Azov to the south and southeast. Kyiv is the nation's capital and largest city, followed by Kharkiv, Odesa, and Dnipro. Ukraine's official language is Ukrainian.

Humans have inhabited Ukraine since 32,000 BC. During the Middle Ages, it was the site of early Slavic expansion and later became a key centre of East Slavic culture under the state of Kievan Rus', which emerged in the 9th century. Kievan Rus' became the largest and most powerful realm in Europe in the 10th and 11th centuries, but gradually disintegrated into rival regional powers before being destroyed by the Mongols in the 13th century. For the next 600 years the area was contested, divided, and ruled by a variety of external powers, including the Grand Duchy of Lithuania, the Kingdom of Poland, the Polish–Lithuanian Commonwealth, the Austrian Empire, the Ottoman Empire, and the Tsardom of Russia.

The Dnieper or Dnepr (/(də)ˈniːpər/ (də-)NEE-pər), also called the Dnipro. is one of the major transboundary rivers of Europe, rising in the Valdai Hills near Smolensk, Russia, before flowing through Belarus and Ukraine to the Black Sea. Approximately 2,200 km (1,400 mi) long, with a drainage basin of 504,000 square kilometres (195,000 sq mi), it is the longest river of Ukraine and Belarus and the fourth-longest river in Europe, after the Volga, Danube, and Ural rivers.

In antiquity, the river was part of the Amber Road trade routes. During the Ruin in the later 17th century, the area was contested between the Polish–Lithuanian Commonwealth and the Russian Empire, dividing what is now Ukraine into areas described by its right and left banks. During the Soviet period, the river became noted for its major hydroelectric dams and large reservoirs. The 1986 Chernobyl disaster occurred on the Pripyat River, a tributary of the Dnieper, just upstream from its confluence with the Dnieper. The Dnieper is an important navigable waterway for the economy of Ukraine and is connected by the Dnieper–Bug Canal to other waterways in Europe. During the 2022 Russian invasion of Ukraine, certain segments of the river were made part of the defensive lines between territory controlled by the Russians and the Ukrainians.

The Crown of the Kingdom of Poland (Polish: Korona Królestwa Polskiego; Latin: Corona Regni Poloniae) was a political and legal concept formed in the 14th century in the Kingdom of Poland, assuming unity, indivisibility and continuity of the state. Under this idea, the state was no longer seen as the patrimonial property of the monarch or dynasty, but became a common good of the political community of the kingdom. This notion allowed the state to maintain stability even during periods of interregnum and paved the way for a unique political system in Poland, characterized by a noble-based parliament and the free election of the monarch. Additionally, the concept of the Crown extended beyond existing borders, asserting that previously lost territories still rightfully belonged to it. The term Crown of the Kingdom of Poland also referred to all the lands under the rule of the Polish king. This meaning became especially significant after the union with the Grand Duchy of Lithuania, when it began to be commonly used to denote the Polish part of the joint Polish-Lithuanian Commonwealth.

The idea of the Crown in Central Europe first appeared in Bohemia and Hungary, from where the model was taken by kings Ladislaus the Short and Casimir III the Great to strengthen their power. During the reign of Louis the Great in Poland, who spent most of his time in Hungary, as well as during the interregnum following his death and the regency during the minority of his daughter Jadwiga, the idea was adopted by the lords of the kingdom to emphasize their own role as co-responsible for the state.

The Polish–Lithuanian union was a relationship created by a series of acts and alliances between the Crown of the Kingdom of Poland and the Grand Duchy of Lithuania that lasted for prolonged periods of time from 1385 and led to the creation of the Polish–Lithuanian Commonwealth, or the "Republic of the Two Nations", in 1569, which ended in 1795 with the Third Partition.

The Pereiaslav Agreement or Pereyaslav Agreement (Ukrainian: Переяславська рада, romanized: Pereiaslavska Rada, lit. 'Pereiaslav Council', Russian: Переяславская рада) was an official meeting that convened for a ceremonial pledge of allegiance by Zaporozhian Cossacks to Russian tsar Alexis (r. 1645–1676) in the town of Pereiaslav in central Ukraine, in January 1654. The ceremony took place concurrently with ongoing negotiations that started on the initiative of Hetman Bohdan Khmelnytsky to address the issue of the Cossack Hetmanate with the ongoing Khmelnytsky Uprising against the Polish–Lithuanian Commonwealth and which concluded the Treaty of Pereiaslav (also known as the March Articles). The treaty itself was finalized in Moscow in April 1654 (in March according to the Julian calendar).

Khmelnytsky secured the military protection of the Tsardom of Russia in exchange for allegiance to the tsar. An oath of allegiance to the Russian monarch from the leadership of the Cossack Hetmanate was taken, shortly thereafter followed by other officials, the clergy and the inhabitants of the Hetmanate swearing allegiance. The exact nature of the relationship stipulated by the agreement between the Hetmanate and Russia is a matter of scholarly controversy. The council of Pereiaslav was followed by an exchange of official documents: the March Articles (from the Cossack Hetmanate) and the tsar's declaration (from Russia).

Belarus, officially the Republic of Belarus, is a landlocked country in Eastern Europe. It is bordered by Russia to the east and northeast, Ukraine to the south, Poland to the west, and Lithuania and Latvia to the northwest. Belarus spans an area of 207,600 square kilometres (80,200 sq mi) with a population of 9.1 million. The country has a hemiboreal climate and is administratively divided into six regions. Minsk is the capital and largest city; it is administered separately as a city with special status.

For most of the medieval period, the lands of modern-day Belarus was ruled by independent city-states such as the Principality of Polotsk. Around 1300 these lands came fully under the Grand Duchy of Lithuania and subsequently by the Polish–Lithuanian Commonwealth; this period lasted for 500 years until the 1792-1795 partitions of Poland-Lithuania placed Belarus within the Russian Empire for the first time. In the aftermath of the Russian Revolution in 1917, different states arose competing for legitimacy amid the Civil War, ultimately ending in the rise of the Byelorussian SSR, which became a founding constituent republic of the Soviet Union in 1922. After the Polish-Soviet War (1918–1921), Belarus lost almost half of its territory to Poland. Much of the borders of Belarus took their modern shape in 1939, when some lands of the Second Polish Republic were reintegrated into it after the Soviet invasion of Poland, and were finalized after World War II. During World War II, military operations devastated Belarus, which lost about a quarter of its population and half of its economic resources. In 1945, the Byelorussian SSR became a founding member of the United Nations, along with the Soviet Union. The republic was home to a widespread and diverse anti-Nazi insurgent movement which dominated politics until well into the 1970s, overseeing Belarus's transformation from an agrarian to an industrial economy.

Warsaw, officially the Capital City of Warsaw, is the capital and largest city of Poland. The metropolis stands on the River Vistula in east-central Poland. Its population is officially estimated at 1.86 million residents within a greater metropolitan area of 3.27 million residents, which makes Warsaw the 6th most-populous city in the European Union. The city area measures 517 km (200 sq mi) and comprises 18 districts, while the metropolitan area covers 6,100 km (2,355 sq mi). Warsaw is classified as an alpha global city, a major political, economic and cultural hub, and the country's seat of government. It is also the capital of the Masovian Voivodeship.

Warsaw traces its origins to a small fishing town in Masovia. The city rose to prominence in the late 16th century, when Sigismund III decided to move the Polish capital and his royal court from Kraków. Warsaw surpassed Gdańsk as Poland's most populous city by the 18th century. It served as the capital of the Polish–Lithuanian Commonwealth until 1795, and subsequently as the seat of Napoleon's Duchy of Warsaw. The 19th century and its Industrial Revolution brought a demographic boom, which made it one of the largest and most densely populated cities in Europe. Known then for its elegant architecture and boulevards, Warsaw was bombed and besieged at the start of World War II in 1939. Much of the historic city was destroyed and its diverse population decimated by the Ghetto Uprising in 1943, the general Warsaw Uprising in 1944, and systematic razing.

Poland, officially the Republic of Poland, is a country in Central Europe. It extends from the Baltic Sea in the north to the Sudetes and Carpathian Mountains in the south, and borders Lithuania and Russia to the northeast; Belarus and Ukraine to the east; Slovakia and the Czech Republic to the south; and Germany to the west. The territory has a varied landscape, diverse ecosystems, and a temperate climate. Poland is composed of sixteen voivodeships and is the fifth most populous member state of the European Union (EU), with over 38 million people, and the fifth largest EU country by land area, covering 312,696 km (120,733 sq mi). The capital and largest city is Warsaw; other major cities include Kraków, Wrocław, Łódź, Poznań, and Gdańsk.

Prehistoric human activity on Polish soil dates to the Lower Paleolithic, with continuous settlement since the end of the Last Glacial Period. Culturally diverse throughout late antiquity, in the early medieval period the region became inhabited by the West Slavic tribal Polans, who gave Poland its name. The process of establishing statehood coincided with the conversion of a pagan ruler of the Polans to Christianity in 966. Under the auspices of the Roman Catholic Church, the dominion became part of the Western world. In 1025, the Kingdom of Poland emerged, and in 1569 it cemented its long-standing association with Lithuania, forming the Polish–Lithuanian Commonwealth. At the time, the Commonwealth was one of Europe's great powers, with an elective monarchy and a uniquely liberal political system. It adopted Europe's first modern constitution in 1791.