On 27 August 1916, Romania entered World War I on the side of the Allies, three of its armies invading Transylvania. The 1st and 2nd Armies invaded the southern part, while the Romanian North Army invaded the northeastern part. Unlike the 1st and 2nd Armies – which had to contend with a German-led counter-offensive (First Battle of Petrozsény, Battle of Brassó) – the North Army faced primarily Austro-Hungarian forces, and as such its campaign hardly had any major battles. After conquering three Hungarian urban districts (including two county capitals), the undefeated North Army was ordered to withdraw due to events outside its control, after inflicting heavier losses than those it had incurred.
Background
The North Army had to cross the greatest distance to accomplish its mission, and as such it had a greater number of soldiers than either of the two other Romanian armies invading Transylvania. Commanding the North Army was General Constantin Prezan, an officer who had studied in France. Prezan was, however, widely regarded as more of a courtier than a serious military officer. Fortunately for the Romanians, his operations officer was Captain Ion Antonescu, a "talented if prickly" individual. Such was the influence of Captain Antonescu that, in his memoirs, General Alexandru Averescu used the formula "Prezan (Antonescu)" to denote Prezan's plans and actions.
Initially, the Romanian North Army was opposed by a single Austro-Hungarian infantry division, the 61st. Part of General Arthur Arz von Straussenburg's 1st Army, it was weak and battered during battles against the Russians, the division amounting to only 3,555 men.
Romanian offensive
The fragile Austro-Hungarian defenses faltered at the first push of the Romanian covering forces. Two regiments from the 15th Brigade of the 8th Division seized Kézdivásárhely (Târgu Secuiesc) on 29 August, the Romanian forces arriving in time to destroy the last train leaving the town. The Austro-Hungarians did manage to blow up a critical railroad viaduct near the border. Although the Romanians had prepared a special commando unit under Lieutenant Ilie Șteflea to cross the border three hours before the start of the invasion and secure the viaduct, the 14th Romanian Infantry Regiment jumped the gun and crossed the border along with the commandos. Having lost the element of surprise, the Romanians alerted the Austro-Hungarians, who blew up the railroad viaduct. In early September, the Romanians reached Csíkszereda (Miercurea Ciuc), capital of Csik County. There, they temporarily halted their offensive and dug in on the east side of the Olt River. Further to the north, the Austro-Hungarians gave up Bélbor (Bilbor) after running out of ammunition. One brigade kept the Romanians largely east of Borszék (Borsec) while gendarmes held their own in the Kelemen (Călimani) Mountains. For the most part, however, Austro-Hungarian soldiers along the border limped towards the Transylvanian interior, accompanied by hordes of German and Hungarian refugees. For instance, over two thirds of the population of Csíkszereda left the town. However, as events would unfold, such behavior would prove exaggerated. In many places, the ethnic-Hungarian civilian population attacked small scattered groups of Romanian soldiers. A fact not merely admitted, but praised in the Hungarian press of the time, such as the newspaper Pesti Napló. In response, the Romanians nowhere took to reprisals, beyond fighting and disarming the civilians who put up active resistance. Wherever the Romanian Army advanced, proclamations were published promising safety and protection to all inhabitants, regardless of nationality. No damage was done by the Romanian armies to the property of civilians, regardless if said property belonged to ethnic-Hungarians (Székelys) or ethnic-Germans (Transylvanian Saxons).