Petru I profited from the end of the Polish-Hungarian union and moved the country closer to the Jagiellon realm, becoming a vassal of king Jogaila of Poland on 26 September 1387. This gesture was to have unexpected consequences: Petru supplied the Polish ruler with funds needed in the war against the Teutonic Knights, and was granted control over Pokuttya until the debt was to be repaid; as this is not recorded to have been carried out, the region became disputed by the two states, until it was lost by Moldavia in the Battle of Obertyn (1531). Prince Petru also expanded his rule southwards to the Danube Delta. His brother Roman I conquered the Hungarian-ruled Cetatea Albă in 1392, giving Moldavia an outlet to the Black Sea, before being toppled from the throne for supporting Fyodor Koriatovych in his conflict with Vytautas the Great of Lithuania. Under Stephen I, growing Polish influence was challenged by Sigismund of Hungary, whose expedition was defeated at Ghindăoani in 1385; however, Stephen disappeared in mysterious circumstances.