 Complete the following phrase:  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 