The Battle of Transylvania was a battle of World War I which was fought from 27 August to 16 October 1916 when Romania launched an offensive to seize Transylvania from Austria-Hungary at the start of the Romanian Campaign of the Eastern Front. Romania joined the Entente alliance in August 1916 despite previously being a tacit ally of the Central Powers, and the Romanians executed a 1913 battle plan to send 75% of their army (420,000 troops) to occupy Transylvania and the Banat, while 72,000 Romanian troops would be sent to prepare for a potential Bulgarian attack in Dobruja. While the Romanians overestimated the number of Austro-Hungarian Army troops defending Transylvania (anticipating 70,000 Austro-Hungarians, but instead facing half of that number), they underestimated the number of reinforcements the Central Powers could deploy to the region, anticipating facing 100,000 Austro-Hungarian and German reinforcements rather than the 300,000 who eventually materialized.
At the start of the offensive, the Romanians captured the vital coal-mining center of Petrozseny (Petrosani) on 29 August, defeating the Hungarian coal-miner battalions and inflicting heavy losses. On 30 August, the Romanians also took the major city of Nagyszeben (Sibiu). Meanwhile, in the southeast, the Romanians took Brasso (Brasov) after advancing through five mountain passes, and, in the north, the Romanians' North Army made rapid progress as the Austro-Hungarian armies disintegrated. However, in mid-September, German reinforcements commanded by Erich von Falkenhayn arrived in Transylvania to reinforce their struggling Austro-Hungarian allies. The Romanian offensive ground to a halt after the Bulgarians attacked Dobruja, and, while the initial German advance in the region was resisted by the Romanians at the bloody Battle of Fogaras, the Romanians were gradually forced to fall back. By mid-October, the three Romanian armies had retreated to the Carpathians while avoiding destruction, although 200,000 civilians were internally displaced as a result of the war for Transylvania. The Romanians were soon forced to fight to defend their own territory, but the near-collapse of Austria-Hungary's military resulted in Emperor Wilhelm II of Germany assuming supreme military command of the Central Powers. The Germans were compelled to shift their attention to the east as a result of the Transylvania campaign, halting their offensive in France at the Battle of Verdun and transferring units to Romania. Falkenhayn, who had originally been in command at Verdun, was replaced as Chief of Staff of the Imperial German Army by Paul von Hindenburg and Erich Ludendorff (the Third Supreme Command) due to the Central Powers' military setbacks, although he was given command of the campaign to subdue Romania.