Romanian troops during the Battle of Mărăşeşti, 1917.
The Entente had been unable to help Serbia and Montenegro, and so had lost its hold on the central Balkan region. The defeat of Serbia in 1915 gave the Central alliance temporary mastery over the Balkans, opening up a land route to Istanbul, thus allowing the Germans to re-supply the Ottoman empire for the rest of the war. By early 1916, only Greece and Rumania remained neutral.
The key event in 1916 was Rumania’s decision to enter the war on the side of the Entente. Offered the Austro-Hungarian regions of Transylvania and Bukovina, and spurred on by the success of the summer 1916 Brusilov offensive, Rumania entered the war on 27 August 1916. This would prove to be her undoing as her 800,000- strong army had little modern weaponry, a poor domestic arms industry, a badly trained officer corps and supply lines dependent on imports from Russia. When Rumania’s army went into action on 27–28 August 1916, the Brusilov offensive had lost momentum. Exposed to possible attacks from Bulgaria in the south and Austria-Hungary in the west, the Rumanians deployed their First, Second and Fourth Armies along the Carpathian Mountain passes and invaded Transylvania, while their Third Army took up positions along the river Danube and in the Dobrudja facing Bulgaria. A rapid response by the Austro-Hungarians, stiffened by German, Bulgarian and Ottoman reinforcements, checked the Rumanians in Transylvania, before they launched a counter-attack.
In early September 1916, Central alliance forces led by the German general August von Mackensen advanced from the south into the Dobrudja, taking the towns of Cernavoda and Constanpa, threatening the Rumanian capital, Bucharest. Another German-led force, commanded by Erich von Falkenhayn, penetrated the Vulcan pass in the Carpathians and engaged Rumanian forces along the river Argerel in late November and early December 1916. Meanwhile, on 23 November, Mackensen switched his focus of operations to Wallachia, crossing the Danube at Sistove (Svishtov) and turning the Rumanian positions facing Falkenhayn along the river Alt (Olt). On 6 December 1916, Mackensen and Falkenhayn’s forces entered Bucharest. As with the Serbs in 1915, the Rumanians were forced into a hasty retreat, this time to the safety of Russian-held Moldavia. The front then stabilised along the river Sereth (Siret). The lack of a coordinated Entente strategy contributed to Rumania’s rapid defeat. An Anglo-French attack from Salonika to help Rumania faltered and, once again, an ally in the Balkans was defeated. The remnants of the Rumanian army, with French help, rebuilt itself into a fighting force of some 500,000 men and counter-attacked in the summer of 1917, culminating in the battle of Mărăşeşti in August 1917. Rumanian forces simply could not sustain such battles and, with the news of the Russian revolution, Rumania entered peace talks with the Central alliance culminating in the Treaty of Bucharest (7 May 1918), a harsh settlement that turned Rumania into a vassal state. On 10 November 1918, with Austria-Hungary and Bulgaria effectively defeated, Rumanian forces again took the field, thus staking Rumania’s claim at the peace talks.