In World War I he participated as a Class I Captain. During Battle of Mojkovac he commanded the Rovač Battalion of the Kolašin Brigade, where he showed great courage and ability to command. After the capitulation of Montenegro in 1916, Vlahović, then captain of the Montenegrin army, and his brother Toško Vlahović, a student of philosophy escaped capture. He did not recognize the capitulation and defected with his brother in the spring and formed a resistance group of twenty people, attempting to harass occupational forces.