Battle of Transylvania

The Battle of Transylvania was the first major operation of the Romanian Campaign during World War I, beginning on 27 August 1916. It started as an attempt by the Romanian Army to seize the disputed province of 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, and a successful German and Austro-Hungarian counterattack after September 18 eventually forced the Romanian Army to retreat back to the Carpathians by late November.

Background
Before the war, the Kingdom of Romania was an ally of Austria-Hungary. However, when war broke out in 1914, Romania pledged neutrality, claiming that Austria-Hungary had started the war and thus Romania had no obligation to join it. Romania eventually joined the Entente, on the condition that the Allies recognise Romanian authority over Transylvania. The province had a Romanian majority, but it had been under Austrian rule since the 17th century (since 1867 by Austria-Hungary). The Allies accepted the terms, and Romania declared war on Austria-Hungary on 27 August.

Romanian offensive (27 August – 15 September)
On the night of 27 August, three Romanian armies crossed the largely undefended Carpathian passes, meeting only sporadic resistance by Austro-Hungarian border units. The Romanian plan (Hypothesis Z) called for a rapid advance to the strategically important Mureş River. Romanian units advanced slowly, taking Braşov on 16 August, crossing the Olt River on 4 September and reaching the outskirts of Hermannstadt (modern-day Sibiu) by mid-September. Three Russian divisions also arrived in Northern Romania, but they suffered from supply shortages and had little overall effect on the fighting. However, by this time Bulgaria declared war on Romania and captured Turtucaia fortress, which, combined with German and Austro-Hungarian reinforcements in Transylvania, led the Romanian High Command to suspend the offensive. Several units were moved from Transylvania to Southern Romania, and the remaining troops switched to a defensive strategy. From this point on, the Romanian Campaign would take a turn for the worse for the Allies.

Central Powers Counter-offensive (18 September – 29 November)
In the meantime, Erich von Falkenhayn, recently fired as Chief of Staff, assumed command of the Ninth Army and begun a counterattack against the Romanians. On 18 September, German forces struck the Romanian First Army near Haţeg, forcing them to retreat. Eight days later, the elite Alpen Korps repulsed a Romanian attack on Sibiu, and on 4 October the Romanian Second Army was defeated at Brasov. The Fourth Army, despite little pressure from the enemy, retreated to the mountains. By 25 October, the Romanian troops were routed and withdrew to their prewar positions. For several weeks, the Ninth Army made probing attacks in the mountains to test the Romanian defenses. On 10 November, Falkenhayn launched his main attack on the Vulcan Pass, inflicting heavy losses on the Romanians. By 26 November, the Romanian defenses were shattered and German troops were crossing into Wallachia, closing in on the Romanian capital, Bucharest.

Aftermath
The battle ended in disaster for the Romanians, who failed to take advantage of their numerical superiority and favorable strategic position (Transylvania was practically a massive, poorly-defended bulge in the Allied lines). General Averescu, the Romanian commander-in-chief, reflected that "the offensive, which faced no difficulties, moved slowly, but when difficulties arose, the retreat was precipitous". Falkenhayn's forces secured a position to attack Bucharest from the North; combined with August von Mackensen's offensive from the south, South-West Romania was enveloped in a double pincer attack, and the Romanian Army fled to Moldova. By 6 December, Romania had suffered 250,000 casualties and lost nearly two-thirds of its territory, including its capital.