Complex sentence: With the help of Lithuanians Tartars, and some Russian principalities, in February 1280 he invaded Lublin, crossed the Vistula, and besieged Sandomierz, which managed to resist; thanks to this, Leszek II was able to reunite enough forces to repel the invasion.
Split into simpler sentences: With the help of Lithuanians Tartars and some Russian principalities, in February 1280 he invaded Lublin, crossed the Vistula, and besieged Sandomierz, which managed to resist. Nevertheless, Leszek II was able to reunite enough forces to repel the invasion.