Rustem was a trumpeter in the 2nd Infantry Division of the Royal Serbian Army, the famed "Iron regiment", during the World War I. On 12 September 1916 the Battle of Kaymakchalan, for the Kajmakčalan mountain between the Serbs and the Bulgarian "Rila Division" began. From 26 to 30 September, the top of the mountain changed hands several times, but Bulgarian had an upper hand and it seemed the Serbs will have to withdraw. Out of the blue, the Serbian charge was played by Rustem and Serbian army attacked the surprised Bulgarians who backed off, allowing the Serbs to conquer the mountain and end the battle on 30 September. Commander Dimitrije Milić, who didn't give the charge order, summoned Rustem to be court-martialed for disobedience but instead, after Rustem told him "I had to do it, whether you shot me or not", he awarded him a medal. Inspecting the battlefield later, Milić discovered a barrel of rum, but it is not known whether Rustem knew that the Bulgarian soldiers were drunk. In 1917, the news of horrible Bulgarian reprisals after the Toplica Uprising (over 20.000 dead) in occupied southern Serbia, reached the Serbian army Salonika Front. Many soldiers originated from the region, including Rustem. He sneaked to the closest Bulgarian position, listened and memorized their trumpet sounds. Days later, with two soldiers, he crawled to the Bulgarian position during the night and while the soldiers threw grenades, he played the Serbian charge. Awaken, Serbian army run forward on an unplanned charge. Rustem then sneaked back and played Bulgarian retreat. Confused, Bulgarian army withdrew and Serb took the position. Tomorrow, Rustem was awarded medals from both prince-regent Alexander and the French general Louis Franchet d'Espèrey. Among his medals were the Order of Karađorđe's Star and the Albanian Commemorative Medal.