The Polish-Ottoman War of 1620-21 was the first conflict between Poland-Lithuania and the Ottoman Empire for ninety years. During that period tensions along the border had been raised by Cossack raids into Ottoman territory, but without triggering a war. Tension rose dramatically in 1618-19. In 1618 Osman II assumed power in the Ottoman Empire, aged only 14. He was looking for a chance to win military glory. He was given his chance in 1620, when Gratiani, the ruler of Moldavia, rebelled against Ottoman rule and called for Polish assistance.
Gratiani promised to raise an army 25,000 strong to support the Polish-Lithuanians. In response Hetman Stanislas Zolkiewski led an army of around 8,000 men south into Moldavia. There he was joined by a tiny Moldavian force, only 600 strong.
The combined army was attacked by a somewhat larger Ottoman army near Cecora (or Tutora). The first attack, on 18 September (sometimes referred to as the battle of Jassy), was beaten off, but a running fight developed as the Polish-Lithuanian army attempted to retreat (from 29 September). On 6 October discipline in the Polish-Lithuanian army collapsed. The army was annihilated. Zolkiewski was killed and his severed head taken to the Sultan.
The Poles restored the situation in 1620. A much larger army was raised and send south under the command of Hetman Chodkiewicz. This army was perhaps 75,000 strong (including 40,000 Cossacks), but it was still outnumbered by the Ottoman army, now commanded by Osman II in person. Chodkiewicz fortified his camp, and for five weeks resisted all Ottoman attacks. Finally, he launched a counter-attack using the Polish hussars, wining a minor victory over Osman (battle of Chocim).
In the aftermath of Chocim, Osman II negotiated a peace treaty. The Poles agreed to restrain the Cossacks, Osman promised to stop Tartar raids into Poland. The brief war had disastrous results for both combatants. Gustav Adolf of Sweden invaded Estonia, taking advantage of the absence of the Polish-Lithuanian army in the far south, securing control of much of Livonia. In 1622 Osman was deposed by a Janissary revolt and replaced by his uncle Mustafa, who was almost immediately overthrown in his own turn in favour of Murad IV.