The Principality of Moldavia was a Polish fief since the Middle Ages and Sigismund aimed at securing that despite the growing threat from the south.
With the Ottoman influence and power on the rise, the Sultan aimed at expanding the Ottoman Empire westward.
Furthermore, the Ottomans favoured the fertile steppes of Moldavia, Ruthenia and "Polish Ukraine".
The Ottoman–Habsburg wars, which lasted almost two centuries, were also a sign of the Sultan's desperation to rule mainland Europe.
Sigismund was anxious to help Habsburgs and was promised territorial gains for Poland in return for his assistance.
He sent in an army consisting of mercenaries from the wars in Russia to the Principality of Moldavia, which sparked the Polish–Ottoman War.

In 1620 the Polish forces were defeated at Cecora and Hetman Żółkiewski perished during the battle.
In 1621 a strong army of Ottomans, led by Osman II, advanced from Edirne towards the Polish frontier.
The Ottomans, following their victory at Cecora, had high hopes of conquering southern part of Polish–Lithuanian Commonwealth, including Podolia, Volhynia and parts of Lesser Poland.
Approximately 160,000 men besieged the Khotyn Fortress in September 1621 but were defeated at the Battle of Khotyn by a Polish garrison counting no more than 50,000 soldiers.
During the siege Hetman Jan Karol Chodkiewicz died of exhaustion and illness in the camp.

The Treaty of Khotyn was signed on 9 October 1621 which resulted in no territorial gain or loss, but Sigismund was to relinquish his claims on Moldavia and the Ottoman Empire was prevented from marching deep into Poland.
Sultan Osman himself was not fully satisfied with the war's outcome and blamed the janissaries.
His wish and plans to modernize the army, which was blamed for the defeat, were however opposed by the tradition-minded janissary units.
That opposition resulted in the 1622 rebellion in which Osman II was deposed and strangled.
The Principality of Moldavia was a Polish fief since the Middle Ages and Sigismund aimed at securing that despite the growing threat from the south. With the Ottoman influence and power on the rise, the Sultan aimed at expanding the Ottoman Empire westward. Furthermore, the Ottomans favoured the fertile steppes of Moldavia, Ruthenia and "Polish Ukraine". The Ottoman–Habsburg wars, which lasted almost two centuries, were also a sign of the Sultan's desperation to rule mainland Europe. Sigismund was anxious to help Habsburgs and was promised territorial gains for Poland in return for his assistance. He sent in an army consisting of mercenaries from the wars in Russia to the Principality of Moldavia, which sparked the Polish–Ottoman War.
In 1620 the Polish forces were defeated at Cecora and Hetman Żółkiewski perished during the battle. In 1621 a strong army of Ottomans, led by Osman II, advanced from Edirne towards the Polish frontier. The Ottomans, following their victory at Cecora, had high hopes of conquering southern part of Polish–Lithuanian Commonwealth, including Podolia, Volhynia and parts of Lesser Poland. Approximately 160,000 men besieged the Khotyn Fortress in September 1621 but were defeated at the Battle of Khotyn by a Polish garrison counting no more than 50,000 soldiers. During the siege Hetman Jan Karol Chodkiewicz died of exhaustion and illness in the camp.
The Treaty of Khotyn was signed on 9 October 1621 which resulted in no territorial gain or loss, but Sigismund was to relinquish his claims on Moldavia and the Ottoman Empire was prevented from marching deep into Poland. Sultan Osman himself was not fully satisfied with the war's outcome and blamed the janissaries. His wish and plans to modernize the army, which was blamed for the defeat, were however opposed by the tradition-minded janissary units. That opposition resulted in the 1622 rebellion in which Osman II was deposed and strangled.
Sigismund III Vasa