The Battle of Transylvania was the first major operation of Romania during World War I, beginning on 27 August 1916. It started as an attempt by the Romanian Army to seize Transylvania, and potentially knock Austria-Hungary out of the war. Although initially successful, the offensive was brought to a halt after Bulgaria's attack on Dobruja. Coupled with a successful German and Austro-Hungarian counterattack which started in mid-September, the Romanian Army was eventually forced to retreat back to the Carpathians by mid-October. The Romanian armies however managed to escape the Central Powers' attempts to completely destroy them. The Battle of Transylvania also caused the replacement of the chief of staff of the German Army and the shifting of German attention to the region, causing German offensive operations at Verdun to cease.
Background
Although bound by the pre-war Triple Alliance to the Central Powers, Romania instead joined the Triple Entente in August 1916, following the signing of the Treaty of Bucharest.
Officers from the Romanian General Staff began the planning of a potential invasion of Hungary as early as 1913, following Romania's successful expansion at Bulgaria's expense during the Second Balkan War. A draft document appeared in the summer of 1914, but Bulgaria's entry into the war on the side of the Central Powers in 1915 forced a major revision of the initial campaign plan. The final version, known as Hypothesis Z, was conceived by General Vasile Zottu. Hypothesis Z called for the Romanian occupation of Transylvania and Banat, using three armies (1st, 2nd and Northern) which amounted to a total of 420,000 soldiers (75% of Romania's field force). The 3rd Army, numbering 72,000 soldiers, was tasked with holding off a potential Bulgarian attack. Hypothesis Z envisioned an Austro-Hungarian opposing force numbering 70,000, but in reality, the actual number was less than half of the Romanian estimate. Another inaccurate Romanian estimate was the number of reinforcements that the Central Powers could deploy to the region: 100,000 compared to the actual number of 300,000.
In July 1916, the Austro-Hungarians began preparations for an eventual defense of Transylvania. By early August, enough units had been gathered in order to form an army, the 1st. On 7 August, this new army was put under the command of General Arthur Arz von Straussenburg. Arz's army had four weak divisions (30-35 battalions) and around 100 artillery pieces in 13 batteries. After the Romanian invasion commenced, the Austro-Hungarian 1st Army was reinforced with one cavalry division, one Reserve Hussar Brigade, one Honvéd infantry division and four battalions.
Battle
South-Western Front
The Romanian 1st Army, commanded by General Ioan Culcer, crossed the border at the Surduc and Vulcan Passes and occupied Petrozsény (Petroșani) — the vital Transylvanian coal-mining center — on 29 August, easily defeating the Hungarian coal-miner battalions and inflicting heavy casualties. The 11th Infantry Division (General Ioan Muică) subsequently continued its advance Northwards, stopping 15 mi (24 km) South of Hátszeg (Hațeg). Although Muică waited for the rest of his Corps as planned (2nd Division and 1st Cavalry Brigade), this allowed the Austrians time to regroup. Sixty-five miles to the East, Culcer's main force — the Olt-Lotru Group — entered the
Turnu Roșu Pass and advanced towards Nagyszeben (Sibiu). After sporadic shootings all throughout the night of 27/28 August, the Romanians took the village of Boica at the head of the Pass. The Lotru Group reached Nagytalmács (Tălmaciu) on 29 August, and subsequently regrouped there before advancing a few more miles North towards Nagyszeben (Sibiu), stopping at Vesztén (Șelimbăr). The advancing Romanians also blocked the important railroad linking Brassó (Brașov) and Nagyszeben (Sibiu). The Austrian commander at Nagyszeben (Sibiu), Colonel Arnold Barwick, sent an armored train to investigate, but this was set ablaze by the Romanians and rendered unserviceable. Nagydisznód (Cisnădie) was taken by the Romanians on 30 August. However, instead of advancing into Nagyszeben (Sibiu), the Romanians started to dig in South of the city during the night of 30 August. Nonetheless, Nagyszeben (Sibiu) was declared an open city and surrendered to the commander of the 23rd Division, General Matei Castris. Castris accepted the surrender, and neither side occupied the city, as the Romanians made no effort to occupy it and the Austrians withdrew. The 1st Army advanced very little throughout September. Its Eastern flank continued to dig in just south of the city, reinforcing its defense lines, while its Western flank moved into the Bănița (Merișor) valley. It inched northwest along the valley, but never left it. The Romanian forces stopped upon running into Austrian and German defenders, coupled with the threat of German forces disembarking at Karánsebes (Caransebeș).
By mid-September, the German 9th Army, under the overall command of Field Marshal Erich von Falkenhayn, was deployed to the region. However, the first German unit to arrive in Transylvania was the 187th Infantry Division, under Major General Edwin Sunkel. Its 187th Regiment was deployed to the area on 5 September. The Germans and Austrians began advancing on 14 September, and on the 18th they took Petrozsény (Petroșani). On 23 September, a skirmish between Germans and Romanians resulted in 107 Germans killed or wounded and 100 Romanians killed plus 23 more captured. The Battle of Nagyszeben (Sibiu) began at the dawn of 26 September, with an attack by the German 9th Army. Its forces in the area were commanded by General Hermann von Staabs, and included the Hungarian 51st Honvéd alongside two German units (187th Division and 76th Reserve Division). The German plan was to isolate the Romanians and cut off the escape route through the Red Tower Pass. The German attack on the 26th ran into stiff resistance, and failed. After amassing his artillery, von Staabs attacked again on the 27th, driving the Romanians towards the mouth of the Red Tower pass by nightfall. Twenty-four hours later, von Staabs had pushed the Romanians into the Pass. German Alpine Corps were deployed to both ends of the Pass on the 26th. The Romanian 48th Infantry Regiment was decimated by German machine gun fire when deployed against the German positions at the North of the Pass. Subsequently, Romanian forces in the Pass moved to its Eastern side, out of the range of German machine guns. Lacking heavy field artillery, the Germans could not completely block the Pass. The Romanians managed to drive off the Germans from the North of the pass, and temporarily from the South of it, but the latter soon returned. The Romanians began their retreat through the pass in earnest on the 28th, through the bullets of the Alpine Corps and German barricades of dead horses and wagons, which had to be blasted away by artillery. In the final hour of the battle, the Romanians made their way through machine gun fire towards the buildings which served as the firing points for the Alpine Corps at the South of the Pass, and bayoneted to death everyone inside. Then, during the afternoon of the 29th, the Romanian forces made it out of the pass and reached safety. Despite the heroic efforts of the Alpine Corps, the bulk of the Romanian force had escaped. The Germans captured the relatively small number of 3,000 prisoners. Although the bulk of the Romanian 1st Army had retreated from Transylvania, more fighting was yet to come. After the Germans captured Petrozsény (Petroșani), they shifted four of their six battalions in the city to contribute to the Battle of Nagyszeben (Sibiu). Ethnic Romanian sympathizers living in the region made General Culcer aware of this, and thus the Romanians counterattacked and re-captured Petrozsény (Petroșani) on the 25th of September. In response, Falkenhayn deployed the German 301st Division under General Johannes von Busse, which arrived in the region on the 28th, linking up with the 2nd Austro-Hungarian Mountain Brigade. The Central Powers commenced their attack on the 30th, capturing Petrozsény (Petroșani) and the Surduc Pass by 5 October after hard fighting. This time however, the Romanians succeeded in destroying the vital coal mines before retreating.
South-Eastern Front
The Romanian 2nd Army, commanded by General Alexandru Averescu, converged on Brassó (Brașov) through five mountain passes. It was opposed by the 71st Infantry Troop division, commanded by General Anton Goldbach. At the Alsótömös (Timișu de Jos), the Székelys' 1st Battalion, supported by an armored train, held its ground for an entire day, only being pushed back on the night of 28 August. In the afternoon of the 29th, the Romanians closed on Brassó (Brașov), their artillery destroying the last train. After local ethnic Romanians surrendered the city to the commander of the 6th Infantry Regiment, Romanian units entered Brassó (Brașov) at around 5 pm and paraded towards the municipal plaza. At the start of September, the 3rd Division inched west along the Olt river towards Fogaras (Făgăraș), reaching Persány (Perșani). The 4th Division occupied Földvár (Feldioara) on 30 August. The 6th Division occupied Sepsiszentgyörgy (Sfântu Gheorghe) on 6 September. The 2nd Army crossed the Olt River at its Southern part on 15 September, then attacked on a line from Fogaras (Făgăraș) to Homoród (Homorod), threatening the juncture between two German units. A breakthrough there would have, at the very least, prevented the German 9th Army from advancing on Nagyszeben (Sibiu). However, the 2nd Army's new commander, General Grigore C. Crăiniceanu, was unaware of just how close he was to rupturing the enemy lines and smash open the front. A Central Powers counterattack stopped the plodding Romanian forces. Crăiniceanu's timidity stemmed partly from the fact that the Romanian General Staff had recently stripped his army of half of its strength, shifting three of its divisions to the Dobruja front. The 2nd Army began to advance northwest again after 10 days. The town of Fogaras (Făgăraș) was taken by the 2nd Army on 16 September. The 2nd Army started advancing again on 29 September, pushing back the Honvéd 2nd Cavalry Division and reaching Székelykeresztúr (Cristuru Secuiesc), a dozen miles from Segesvár (Sighișoara).