In 1397 both rulers signed a treaty in which Tokhtamysh confirmed Vytautas as a rightful ruler of Ruthenian lands that were once part of the Golden Horde, and now belonged to Lithuania, in exchange of military assistance. It is possible that the treaty required Vytautas to pay tribute from these lands once the khan regained his throne. Vytautas probably was planning to establish himself as overlord in Tatar lands. This joint expedition, supported by Polish volunteers under Spytek of Melsztyn, was defeated on 12 August 1399 in the Battle of the Vorskla River by forces led by Temür Qutlugh and Nogay emir Edigü. The defeat was disastrous, ending Vytautas's ambitious policy in the Pontic steppe and forcing Tokhtamysh to seek refuge again, this time in Siberia. About twenty princes, among them Andrei of Polatk and Dmitry of Briansk were killed, along with many prominent lords, like Spytko.