[4][5] The invasion began in late 1259, after a powerful Mongol army had been sent to the Kingdom of Galicia–Volhynia in order to punish King Daniel of Galicia for his independent actions.

To weaken Daniel's position, the Golden Horde decided to attack his allies, Hungarian King Béla IV, and Duke of Kraków, Bolesław V the Chaste.

Altogether, Mongol forces under Boroldai were 30,000 strong, with Ruthenian units of King Daniel of Galicia, his brother Vasilko Romanovich, Kipchaks and probably Lithuanians or Yotvingians.

[citation needed] The Mongolian army concentrated near Chełm, and after capturing Polish towns east of the Vistula, the invaders appeared at Sandomierz (early December 1259).

Boroldai ordered Ruthenian auxiliary units to besiege and capture the city, while main Mongol forces marched westwards toward the Holy Cross Mountains.

Ruthenian princes, which took part in the siege, advised Piotr of Krepa to accept Mongol offers, and abandon Sandomierz, in exchange of safe passage for all residents of the city.

Finally, facing hunger and epidemics, the Poles left Sandomierz on February 2, 1260; the Mongols broke their promise and massacred the civilians and the defenders.