At the end of the April War, Mašera, along with his fellow Lieutenant Milan Spasić, scuttled the destroyer Zagreb in the Bay of Kotor near Tivat to prevent its capture by the Italian Royal Navy (Regia Marina Italiana).

After the end of World War I, his family fled from the Italian-administered Julian March to the Kingdom of Serbs, Croats and Slovenes (Yugoslavia), in order to escape the violent policies of Fascist Italianization.

In Ljubljana, Masera received primary and secondary education before enrolling in the Naval Military Academy (VII class) in Dubrovnik.

[1] At the outbreak of the April War in 1941, Sergej Mašera was a lieutenant on the destroyer Zagreb stationed in Dobrota on the Bay of Kotor.

At that time, Zagreb (together with the destroyers Belgrade and Dubrovnik) was one of the most recent Yugoslav Navy ships, and was therefore the prime target of an air attack of five Regia Aeronautica bombers on the 6th of April.

Two days later, the demoralized Yugoslav Royal Army asked for a truce, and the crews of all ships stationed in the Bay of Kotor were instructed to cease fire and surrender peacefully.

In 1968, a French film entitled Flammes sur l'Adriatique (also called Adriatic Sea of Fire) [4][5] commemorated the destruction of Zagreb and the heroism of Mašera & Spasić.

Awareness of the men and their actions began to grow during this time—due, in part, to the film, but also to changing opinions about the period during and after World War II.

In similar fashion, the city park in the Montenegrin town Tivat contains a monument to the events that took place in the nearby Bay of Kotor.