Actually, making the choice was a no-brainer, because by that time Adolf Hitler already had the necessary convictions (necessary to make this choice that is) instilled in him by Herr Professor Pötsch and by the general German nationalist environment.
Ten years later, Adolf Hitler wrote in Mein Kampf:
“As a logical consequence of these convictions, there arose in me a feeling of intense love for my German−Austrian home and a profound hatred for the Austrian State.”
As an Austrian citizen, he was subject to be drafted to the Imperial Army when he reached the age of twenty-one. However, he simply ignored the draft and did not register for the compulsory military service in fall of 1909 (the service was to begin in spring of 1910).
For three years he successfully avoided the military authorities which was not that difficult for an essentially homeless man. However, in the spring of 1913 he apparently decided that he had enough of everything Austrian and after receiving his share of his father’s inheritance, left for Munich. For his beloved Germany.
However, it did not prevent the Austrian military authorities from successfully pursuing him (apparently he underestimated the level of cooperation between Austrian and Bavarian authorities in these matters).
The Austrians located him and forced him to come to Salzburg on 5 February 1914 for medical assessment. The verdict was unexpected given that no one noted any serious problems with his health – unfit for military service. So he returned to Munich.
Given the fact that by that time he already received a sizable inheritance from his father and made a decent living painting and selling postcards, bribe was the most logical explanation.
Especially given the fact that just six months later, in August 1914, he voluntarily enlisted in the Bavarian Army and the German medical assessment produced exactly the opposite verdict – fit for military service.
There was a little problem, however – he was the Austrian, not German citizen and thus had no right to volunteer for the military service in Bavaria. Bavarian military authorities later claimed it was an administrative error but again, a bribe was again a much more logical explanation.
However, the officers on a medical board could just have been ardent German nationalists so they simply made an exception for a kindred soul.
Anyway, in late August of 1914 he was posted to the 1st Company of the Bavarian Reserve Infantry Regiment 16 which in September became a part of the 6th Bavarian Reserve Division. As a reserve division, it consisted mainly of recalled reservists, but included a considerable number of volunteers.
One of which was newly minted Private Adolf Hitler.