The Hypothesis Z (Romanian: Ipoteza Z) was the name of Romania's war plan for World War I. It was based on an offensive against the Austro-Hungarian EmpireinTransylvania and a simultaneous defense of the country against Bulgarian attacks in the south. Once Russian forces arrived in Dobruja, the Romanian Army would launch another offensive against Bulgaria. The Hypothesis Z was definitively developed and finished in July 1916. Its application in the war, to which Romania entered in August 1916, was unsuccessful, but its main objective, to achieve the unification of the Romanian nation, was realized anyway after the war.
When World War I began, the Chief of the Romanian General Staff began studying and elaborating several war plans with the objective of attending state interests in the new political and military scenario of Europe. These war plans were called "operation plans" or "hypotheses", and had several variants with the intention of adapting to any possible military situation. These plans were developed with the idea that a war against Romania could exist in three directions: south, northwest and east. Those operations related to the south of the country (against Bulgaria) were assigned the letter A (Hypothesis A), those related to the northwest (against Austria-Hungary) had the letter B (Hypothesis B), and those related to the eastern border (the one with Russia) had the letter C (Hypothesis C). These plans had many subvariants, which were differentiated through numbers (i.e. Hypothesis A1, Hypothesis A2, etc.).[1]
Between the years 1914 and 1916, subvariants of these hypotheses were prepared and studied. Initially, they treated a possible war with Russia, but they eventually switched mainly on the invasion of Austro-Hungarian territory and the defense of the border with Bulgaria. This plan came to be known as the Hypothesis B3. Between March and April 1916, the last studies of this subvariant were performed and it was agreed to be the one used in the case of war. Thus, it was determined that Romania would launch an offensive against Austria-Hungary and defend itself from Bulgaria, the Hypothesis B3 earning the name of "Hypothesis Z".[1] Romania was only supposed to defend itself from Bulgarian attacks until the arrival of Russian troops in Dobruja, after which it would begin an offensive into Bulgarian territory.[2]
The Chief of the Romanian General Staff finalized the Romanian war plan in July 1916 and began distributing it to Romanian commanders. Through the Hypothesis Z, Romania intended to conquer all territories from Austria-Hungary populated with Romanians and the unification of the Romanian nation.[1] Hypothesis Z started its application after the entrance of Romania into the World War I in August 1916.[2][3] According to the Hypothesis Z, the Northern Army, the First Army and the Second Army (about 75% of the Romanian Army) were to begin an offensive in Transylvania towards Brașov, Târgu Mureș, Cluj-Napoca and finally Budapest. On the other hand, the Third Army (the remaining 25%) had to stay on the Danube and in Dobruja to defend Romania from Bulgarian attacks. The Fifth Army Corps [ro], constituting the army reserve, would initially be placed in the area near Bucharest and then move to Transylvania. The Romanian Danube Flotilla was placed near the portofTutrakan in Dobruja.[1][3] The total number of soldiers that were to participate was 562,847, 420,324 for the Transylvanian front and 142,523 for the Bulgarian one.[3] In the end, even though the Romanian Army faced initial defeats during the war, the aim of the Hypothesis Z, the national unification of the Romanians, was achieved in its aftermath through the Great Union.[1][2]