The government of the Ukrainian People's Republic, with mounting attacks on its territory since early 1919, had lost control over most of Ukraine, which was controlled by several disparate powers: Denikin's Whites, the Red Army and pro-Soviet formations, the Makhnovist Partisan Army claiming significant territory, the  Kingdom of Romania in the southwest, Poland, and various bands lacking any political ideology.
The city of Kiev had undergone numerous recent changes of government.
The Ukrainian People's Republic was established in 1917; a Bolshevik uprising was suppressed in January 1918.
The Red Army took it in February 1918, followed by the Army of the German Empire in March; Ukrainian forces retook the city in December.
During February 1919 the Red Army regained control; in August it was taken first by Symon Petlura's men and then by Denikin's army.
The Soviets regained control in December 1919.

At the time of the offensive, the forces of the exiled Ukrainian leader Petlura, who formally represented the Ukrainian People's Republic, controlled only a small sliver of land near the Polish border.
Under these circumstances, Petlura saw no choice but to accept Piłsudski's offer to join the alliance with Poland despite many unresolved territorial conflicts between these two nations; on April 21, 1920 they signed the Treaty of Warsaw.
In exchange for agreeing to a border along the Zbruch River, Petlura was promised military help in regaining the Soviet-controlled territories with Kiev, where he would again assume the authority of the Ukrainian People's Republic (UNR).

For the Petlura's acceptance of the Poland's territorial advances it obtained from defeating the West Ukrainian People's Republic (WUNR), a Ukrainian statehood attempt in Volhynia and Eastern part of Galicia, largely Ukrainian populated but with significant Polish minority, Petlura was promised military help in regaining the Soviet-controlled territories with Kiev, where he would again assume the authority of the Ukrainian People's Republic.
The treaty was followed by a formal alliance signed by Petlura and Piłsudski on April 24.
On the same day, Poland and UPR forces began the Kiev Operation, aimed at securing the Ukrainian territory for the Petlura's government thus creating a buffer for Poland that would separate it from Russia.

Following the formal restoration of Ukrainian independence, the Ukrainian state was then supposed to subordinate its military and economy to Warsaw through joining the Polish-led "Międzymorze" federation of East-Central European states, as Piłsudski wanted Ukraine to be a buffer between Poland and Russia rather than seeing Ukraine again dominated by Russia right at the Polish border.
Separate provisions in the treaty guaranteed the rights of the Polish and Ukrainian minorities within both states and obliged each side not to conclude any international agreements against each other.

As the treaty legitimized the Polish control over the territory that the Ukrainians viewed as rightfully theirs, the alliance received a dire reception from many Ukrainian leaders, ranging from Mykhailo Hrushevsky former chairman of the Tsentralna Rada, to Yevhen Petrushevych, the leader of the West Ukrainian People's Republic that was forced into exile after Polish-Ukrainian War.
However, such objections were brushed aside.

The initial expedition in which 65,000 Polish and 15,000 Ukrainian soldiers took part started on April 24, 1920.
The military goal was to outflank the Soviet forces and destroy them in a single battle.
After winning the battle in the South, the Polish General Staff planned a speedy withdrawal of the 3rd Army and strengthening of the northern front where Piłsudski expected the main battle with the Red Army to take place.
The Polish southern flank was to be held by Polish-allied Ukrainian forces under a friendly government in Ukraine.
On May 7, Polish and Ukrainian soldiers entered Kiev.
The government of the Ukrainian People's Republic, with mounting attacks on its territory since early 1919, had lost control over most of Ukraine, which was controlled by several disparate powers: Denikin's Whites, the Red Army and pro-Soviet formations, the Makhnovist Partisan Army claiming significant territory, the  Kingdom of Romania in the southwest, Poland, and various bands lacking any political ideology. The city of Kiev had undergone numerous recent changes of government. The Ukrainian People's Republic was established in 1917; a Bolshevik uprising was suppressed in January 1918. The Red Army took it in February 1918, followed by the Army of the German Empire in March; Ukrainian forces retook the city in December. During February 1919 the Red Army regained control; in August it was taken first by Symon Petlura's men and then by Denikin's army. The Soviets regained control in December 1919.
At the time of the offensive, the forces of the exiled Ukrainian leader Petlura, who formally represented the Ukrainian People's Republic, controlled only a small sliver of land near the Polish border. Under these circumstances, Petlura saw no choice but to accept Piłsudski's offer to join the alliance with Poland despite many unresolved territorial conflicts between these two nations; on April 21, 1920 they signed the Treaty of Warsaw. In exchange for agreeing to a border along the Zbruch River, Petlura was promised military help in regaining the Soviet-controlled territories with Kiev, where he would again assume the authority of the Ukrainian People's Republic (UNR).
For the Petlura's acceptance of the Poland's territorial advances it obtained from defeating the West Ukrainian People's Republic (WUNR), a Ukrainian statehood attempt in Volhynia and Eastern part of Galicia, largely Ukrainian populated but with significant Polish minority, Petlura was promised military help in regaining the Soviet-controlled territories with Kiev, where he would again assume the authority of the Ukrainian People's Republic. The treaty was followed by a formal alliance signed by Petlura and Piłsudski on April 24. On the same day, Poland and UPR forces began the Kiev Operation, aimed at securing the Ukrainian territory for the Petlura's government thus creating a buffer for Poland that would separate it from Russia.
Following the formal restoration of Ukrainian independence, the Ukrainian state was then supposed to subordinate its military and economy to Warsaw through joining the Polish-led "Międzymorze" federation of East-Central European states, as Piłsudski wanted Ukraine to be a buffer between Poland and Russia rather than seeing Ukraine again dominated by Russia right at the Polish border. Separate provisions in the treaty guaranteed the rights of the Polish and Ukrainian minorities within both states and obliged each side not to conclude any international agreements against each other.
As the treaty legitimized the Polish control over the territory that the Ukrainians viewed as rightfully theirs, the alliance received a dire reception from many Ukrainian leaders, ranging from Mykhailo Hrushevsky former chairman of the Tsentralna Rada, to Yevhen Petrushevych, the leader of the West Ukrainian People's Republic that was forced into exile after Polish-Ukrainian War. However, such objections were brushed aside.
The initial expedition in which 65,000 Polish and 15,000 Ukrainian soldiers took part started on April 24, 1920. The military goal was to outflank the Soviet forces and destroy them in a single battle. After winning the battle in the South, the Polish General Staff planned a speedy withdrawal of the 3rd Army and strengthening of the northern front where Piłsudski expected the main battle with the Red Army to take place. The Polish southern flank was to be held by Polish-allied Ukrainian forces under a friendly government in Ukraine. On May 7, Polish and Ukrainian soldiers entered Kiev.
Kiev Offensive (1920)