Russia and Ukraine have a long history, and the Kiev region of Ukraine was once the birthplace of Russia, and for hundreds of years, Russia and Ukraine were separated and merged.
In the 9th century AD, the earliest civilization on the plains of Eastern Europe, Kievan Rus rose, with a population of more than 5 million, the border was unprecedentedly vast, became the largest country in Europe at that time, the ruler was the Grand Duke of Vladimir, and later the Grand Duke of Vladimir married Princess Anna of the Byzantine Eastern Roman Empire, and since then Russia considers itself the orthodox successor of the Roman Empire and calls itself the Third Roman Empire. Kievan Rus' laid the common cultural foundation of Russia and Ukraine today.
In 1654-1667, Russia and Poland fought for Ukraine for a full 13 years, annexing eastern Ukraine, but what about the remaining Ukraine? The story of Ukraine goes like this, in the 1640s the Cossacks of the Dnieper River became increasingly dissatisfied with their treatment. In 1648, the Ukrainian people, under the leadership of the Cossack Khmelnytsky, rebelled against the oppression of the Polish nobility and the Catholic Church, and the Cossacks defeated the Polish advance troops.
Later, after signing several unsatisfactory treaties with the Polish king, he lost the battle against the Polish army, so Khmelnytsky believed that the Cossacks alone would never be able to get the position they wanted in Poland by their own strength. He therefore requested the support of the Tsar of the Muscovite Principality and the transformation of Ukraine into a protectorate of the Muscovite Principality. The Tsar was persuaded to agree to support the Cossacks, which was the case in 1653-1654. Then a long Russo-Polish war began.
During the war, however, Russia controlled only eastern Ukraine, i.e. Ukraine east of the Dnieper River. In fact, the biggest defeat of the Muscovite army in the war was the defeat of the Ukrainians at the Battle of Konotop in 1659, so why did Ukraine later oppose Moscow? Cossacks did not spread throughout Ukraine. The most important thing is to understand the social structure of Ukraine over the years. By the mid-17th century, two political classes had emerged there. One was the large landowners and petty aristocratic classes, lords with serfs and peasants. They were Catholics, even those from the Ukrainian (i.e. Russian) people, and part of the Polish ruling class.
The other was the free Cossacks of the Zaporozhye group, mostly serfs or peasants who had fled from the landlords' homes. There was also a layering within the Cossacks, and the military chiefs of the Cossacks wanted to match the privileges of the nobility, such as Khmelnytsky. The question, therefore, was how best to ensure their class position for the leaders of the Cossack rebellion. Thus, Khmelnytsky's successor, Ivan Wigovsky, returned Ukraine to Poland as early as 1658. Ukraine's very accession to Russia (not to mention "reunification") is wrong. Ukraine sought independence but was forced to sow discord between the powerful.
Other countries are also involved in the conflict – Sweden, Crimea, Turkey. Khmelnytsky also plotted to withdraw from the treaty with Moscow and accept the protection of the Swedish king. In 1655, with the help of the Ukrainian Cossacks, Russia easily occupied most of Poland's territory, occupying Werna and Lublin in the west. Had it not been for the attempt by rival Sweden to occupy Lithuania and the dispersal of Russian forces, Tsar Alexei might have completely overthrown the Polish king. The war between Russia and Sweden began, and many Polish nobles took advantage of this to oppose Russia. At the same time, ukrainian Cossacks abandoned their alliance with Russia and even opposed Moscow.
In 1658, Russia and Sweden reconciled under the status quo, but at the same time, powerful Poland was once again opposed to Russia. The Ukrainian Cossacks split into two parties: the Polish Party and the pro-Moscow Party. During this war, the two parties gradually established the geographical border of the Dnieper River, which later became the dividing line between Russia and Poland. Khmelnytsky's successor, Ivan Wigowski, returned Ukraine to Poland in 1658. According to historical records, Russia and Poland signed the Treaty of Andrewsov in 1667, under which the Muscovite Principality received most of the land on the left bank of Smolensk and Ukraine, including Kiev and the Zaporozhye Islands.
To be sure, in the final stages of the war, Russia and Poland were fighting not so much for Ukrainian territory as for control of Ukrainian territory with Ukrainian barracks. The rapprochement of the two great powers divided a rebellious Ukraine, which was neither completely subservient to Russia nor Poland.