Vratislaus II of Bohemia
When Hildebrand of Sovana, an enemy of the German king, became Pope Gregory VII in 1073, Bolesław II saw in him a natural ally; he started to apply the Pope's reforms in the Archbishopric of Gniezno and commenced negotiations to obtain the royal crown. He spurred the ongoing revolt in Saxony, which had forced Henry IV to retreat from that region (he crushed the revolt at the Battle of Langensalza soon thereafter); the Polish king seized the occasion to launch an invasion against Henry IV's vassal, Vratislaus II of Bohemia, alongside an ally from Grand Prince Vladimir II Monomakh of Kiev.