Matthias II considered Gabriel's attack against Wallachia as treachery, because he regarded Transylvania and the two Romanian principalities as realms of the Hungarian Crown. Zsigmond Forgách, commander-in-chief of Upper Hungary, invaded Transylvania in late June. Although Nagy and the Hajdús under his command supported Forgách, most Protestant noblemen refused to join the invasion. Most Transylvanians regarded the invasion as an unlawful action, and only the Saxons were willing to support Forgách. He and Radu Şerban besieged Szeben, but could not capture it. Gabriel sent envoys to Istanbul, seeking assistance from the Sublime Porte. Nagy and his Hajdú troops deserted Forgách and routed the reinforcements sent to him from Upper Hungary in mid-September. After learning of the arrival of Ottoman troops to support Gabriel, Radu Şerban withdrew from Szeben; this forced Forgách to lift the siege. The Transylvanian army routed the retreating royal troops, capturing hundreds of soldiers.