Denis participated in the disastrous Battle of Mohi on 11 April 1241, from where fled together with king Béla IV to the Dalmatian coast. Denis as royal governor of Dalmatia, took part in the Siege of Zara in 1243. On 30 June 1244, Béla made a peace with the Republic of Venice and he surrendered his supremacy over Zara (today Zadar, Croatia) but he retained the one third of the Dalmatian city's revenues of customs. In 1244, Denis led the royal army against Spalato (today: Split, Croatia), which rebelled against the Hungarian monarch.