By the outbreak of the First World War Gál's career had already made a promising start, with successful performances of his works not only in Austria but also further afield in Germany. The war was an unwelcome interruption, which inevitably destroyed - at least for several years - much of what had been achieved. In 1915 he was drafted into the army, serving first in Serbia. His 'active service' does not appear to have been so active as to stem the flow of his compositions, however. At this time, as also later in his life, his salvation was the ability, developed in his youth through confined domestic circumstances, simply to shut out all external distractions and to concentrate on his own work. As a result a number of significant works emerged even in these disrupted years, including the cantata Vom Bäumlein, das andere Blätter hat gewollt ('The tree that wanted different leaves') of 1916, for alto solo, six-part female chorus and small orchestra, which was immediately published as his Op. 2. His experiences in Belgrade, hearing folk-tunes played in local cafés, served as the basis for his Serbische Weisen ('Serbian Dances') for piano duet (Op. 3), composed in Belgrade in 1916, which were only published after the war, but then on account of their popularity rapidly appeared in various 'pirate' arrangements, making it necessary for the composer to produce his own orchestral arrangement. There followed a string quartet, also completed in 1916, which was given its first performance in Vienna by the Rosé-Quartet in the same year and (unlike two earlier string quartets) sufficiently satisfied the sharper self-criticism of his maturer years as to be finally published in 1924 as his Quartet No. I (Op.16). Erwin Kroll describes it as

"one of the few creations of recent date which really breathe something of the spirit of Schubert. This quartet is Schubertian in its blissful major-minor tonality, its melodic richness, its piquant rhythms and dance-like exhilaration." ['Vom Schaffen Hans Gáls', Simrock Jahrbuch 2, 1929, p.172]

In the autumn of 1917 Gál was transferred to the Polish Carpathians, in the north-eastern part of the Austro-Hungarian empire. Here he was with a construction detachment which had the task of building a mountain railway - fortunately well behind the front line. However, he had more ambitious plans than merely building a railway, as he explained:

"With my poor eyesight I was soon out of the troop that had anything to do with weapons. My rifle had become too dangerous for our own people. Well, in short, I was assigned to the administration troop and was relatively safely housed there. My main occupation at that time was to write an opera." [Quoted from Hans Gál zum 100. Geburtstag. Kulturdezernat der Stadt Mainz, 1986.]

The opera - Der Arzt der Sobeide ('Sobeide's Doctor') - was completed in Italy one year later (see below). Again, it was not his first attempt; as a 20-year old he had already completed - and 'laid aside' - an opera called Der Fächer ('The Fan'), based on a text by Goldoni. The scene of Gál's final war-time activity was South Tirol. When it became clear to him that the war was completely lost and the front was about to break up, he decided, without waiting for orders, to retreat with his company. At the border between Italy and Austria he had to dispose of the company's transport, a cart with horses and oxen. The receipt, duly stamped by the authorities in Bolzano, has been preserved.