During World War I Sondermajer was the only fighter plane pilot from the Kingdom of Yugoslavia to get selected to join Les Cigognes (The Storks), the elite French Air Force unit, fighting on the Western Front, most notably against the German squadron of the Red Baron.

In October 1912 when the First Balkan War broke out and Serbia, Montenegro, Greece, and Bulgaria raised against the Ottoman Empire, twenty years old Tadija Sondermajer left his studies in Germany and immediately rushed back home enlisting in the Serbian cavalry as a volunteer.

In the Summer of 1914, Austro-Hungarian troops invaded the Kingdom of Serbia, Tadija Sondermajer joined the 4th Cavalry Regiment under Lieutenant Colonel Petar Zivkovič, despite heavy loss on both sides, the Serbian army managed to repulse three successive Austro-Hungarian attempted invasions of the country;[3] When Belgrade was liberated after twelve days of occupation on 15 December 1914, Tadija led a cavalry patrol that first entered the city escorting King Peter to church.

The Sondermajer brothers crossed the snowy Albanian mountains in the arduous winter retreat with the rest of the Serbian army and a substantial number of civilians; they settled on the Greek island of Corfu in early 1916.

[5] On Corfu, Tadija and Vladimir entered the air force as an observer on scouting missions over the Eastern Front, completing after several months course a reconnaissance course in Salonika on 22 June 1916.

On one occasion he succeeded in forcing a superior enemy aircraft to escape, for which he received praise from Field Marshal Stepa Stepanović.

During one mission above the enemy line, as he was returning to the base with an empty machine gun, Sondermajer was attacked by three German fighters.

On his return from a second fight on May 21 his SPAD XIII caught fire after being shot, miraculously he was able to land in Flanders, Sondermajer survived the crash of his plane but was severely burned with a leg injury.

Sondermajer is asked to launch a major initiative to replace World War I era aircraft still in service with more modern ones.

[10] At a meeting of the Aero Club, in June 1926, Sondermajer and other members got into an argument with Miloš Crnjanski about the type of planes that the national airline should purchase.

The battle was fought gentlemanly, with minutes taken and in the presence of seconds[b] (Crnjanski was represented by theatre director Branko Gavela and writer Dušan Matić), with trophy weapons borrowed especially for the occasion from the Dunđerski family.

The company pledged to launch the Belgrade-Zagreb line while the state promised to cover three-quarters of the cost per kilometre of flight in cash and in kind.

Aeroput's management opted for this type of aircraft because of the local Ikarbus factory in Zemun, under license from the same French company that produced the Potez 25 planes for the Air Force.

[15] On 5 May 1930 Tadija along Vladimir Striževski, chief pilot of the Company, landed the first passenger plane in Podgorica, Montenegro, on an old airfield near Vrela Ribničkih, the aircraft was a Potez 29-2 biplane with nine seats with the name "Skopje".

[17] In March 1941, the Air Force took over the civilian aviation fleet, a Transport Group was formed, comprising all nine Aeroput planes as well as one government aircraft.

[20] Wehrmacht forces, backed by Italian, Romanian, Hungarian and Bulgarian allies, invaded Yugoslavia, on 22 April 1941, Serbia was placed under the authority of the German Military Administration.

[22] Upon their release, Sondermajer's sons, 17-year-old Stanislav and 13-year-old Mihailo, made their way to Ravna Gora, the highland of central Serbia, and joined the Chetnik resistance, under the leadership of Colonel Draža Mihailović, who was fighting the German occupation.

[27] His wife Milica started campaigning for their release, pre-war friends, who were close to the Communists, and who came to power after the war, could not or did not want to help,[c] after a year she managed to speak to Aleksandar Ranković who promised that Sondermajer would not be executed.

In June 1947 he began working part-time for the construction enterprise Polet, Sondermajer lost property, income and friends and was never again allowed to fly or have any contact with aviation.

[28] Tadija's brother Vladislav Sondermajer was captured by the Nazis and sent to a prison camp in Germany, he never went back to his country, he died in York in 1949.

[29] On 17 June 2017, on the occasion of the 90th anniversary of the founding of Aeroput, a bust of Tadija Sondermajer was unveiled in the city park, on the corners of Palmotićeva and Koste Stojanovića streets in Belgrade, by the representatives of the institutions which supported the project realization: the deputy mayor of Belgrade Andreja Mladenović, the manager of the Directorate of the Civil Aviation Mirjana Čizmarov and the director of Air Serbia Dane Kondić.

[31] An exhibition called "Sondermajer - born Polish, but with a Serbian heart" took place at the Jadar Museum in Loznica in December 2017.