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 . Battle . 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 .