The Kingdom of Romania was neutral for the first two years of World War I, entering on the side of the Allied powers from 27 August 1916 until Central Power occupation led to the Treaty of Bucharest in May 1918, before reentering the war on 10 November 1918. It had the most significant oil fields in Europe, and Germany eagerly bought its petroleum, as well as food exports.
The Flămânda Offensive, which took place during World War I between 29 September and 5 October 1916, was an offensive across the Danube mounted by the Romanian 3rd Army supported by Romanian coastal artillery. Named after the hamlet of Flămânda, the battle represented a consistent effort by the Romanian Army to stop the Central Powers' southern offensive led by August von Mackensen. The battle ended as a tactical victory for the Central Powers.
The Royal Romanian Navy during World War I (1914–1918) was divided into two fleets and fought against the forces of the Central Powers. When Romania entered the war in August 1916, the Romanian Navy was officially divided as follows :
The Battle of Prunaru was a military engagement between German and Romanian forces during the Romanian Campaign of World War I. It resulted in a tactical German victory, but following the heavy Romanian resistance the Germans halted after taking Prunaru. General Constantin Prezan's maneuver group checked the German forces in the region within two days, exposing the left flank of Field Marshal August von Mackensen's Danube Army.
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.
The Yugoslav monitor Drava was a river monitor operated by the Royal Yugoslav Navy between 1921 and 1941. She was originally built for the Austro-Hungarian Navy as the name ship of the Enns-class river monitors. As SMS Enns, she was part of the Danube Flotilla during World War I, and fought against the Serbian and Romanian armies from Belgrade to the lower Danube. In October 1915, she was covering an amphibious assault on Belgrade when she was holed below the waterline by a direct hit, and had to be towed to Budapest for repairs. After brief service with the Hungarian People's Republic at the end of the war, she was transferred to the newly created Kingdom of Serbs, Croats and Slovenes, and renamed Drava. She remained in service throughout the interwar period, but was not always in full commission due to budget restrictions.
Ioan Culcer was a Wallachian-born Romanian military leader and politician. Culcer served as a lieutenant during the Romanian War of Independence (1877–1878) and as a general during the Second Balkan War and World War I. In early 1918, he served as Minister for Public Works in the First Averescu cabinet.
The First Battle of the Jiu Valley was a military engagement during World War I fought between Romanian forces on one side and Central Powers forces on the other. The German offensive, although initially successful, was checked within days and subsequently repulsed by a Romanian counterattack. This was the most conspicuous Romanian victory during the 1916 campaign, given that it was achieved against forces which were superior in artillery and - initially - in numbers as well.
The Orsova Offensive was a military engagement fought between the armies of Romania and Austria-Hungary at the start of the Romanian Campaign in August–September 1916. It ended in a Romanian victory which led to the Romanian occupation of the west bank of the Cserna (Cerna) River for over two months.
The Battle of Báránykút was a military engagement during the Battle of Transylvania, at the beginning of the Romanian Campaign of World War I. It consisted in a German offensive that was successfully repulsed by the Romanians, who then carried out a largely unhindered tactical retreat.
The Romanian Debacle consisted in a series of battles between November and December 1916 which led to the Central Powers conquest of Bucharest, the capital of Romania. Russian forces joined the Romanians at the start of December.
The 1917 Romanian Campaign consisted in three battles between late July and early September 1917, fought between Germany and Austria-Hungary on one side versus Romania and Russia on the other. Romania emerged from this campaign with a slight net territorial gain and won its most important battle during the First World War.
The Battle of the Olt Valley consisted in a prolonged military engagement during World War I between Romanian forces on one side and Central Powers' forces on the other side. Within just under a month, the Central Powers had managed to conquer the 30 miles (48 km)-long Olt Valley from its Romanian defenders.
The Pitești–Târgoviște Retreat was a fighting-withdrawal operation carried out by the Romanian 1st Army in the face of advancing Central Powers' forces during World War I. The retreat lasted from 29 November to 3 December and culminated in a violent battle at Târgoviște, after which the entire Romanian Army started a general retreat towards Moldavia.
The Battle of Brassó was the last major military engagement during the Battle of Transylvania. It took place between 7 and 9 October 1916, between Central Powers forces on one side and Romanian forces on the other side. As Brassó was the second largest city in Transylvania and the largest Transylvanian settlement taken by the Romanians during their August-September offensive in the region, the retreating Romanian forces decided to make a stand there. However, when its defense proved untenable, the Romanian 2nd Army withdrew from the city, leaving behind only 1,175 prisoners. As the Romanians failed to defend Brassó, so did the Central Powers fail to cut off their escape route, enabling the Romanians to regroup and — under better leadership — thwart the German and Austro-Hungarian attempts to break through the mountains in the region throughout the following month.
The Battle of the Eastern Carpathians consisted in a series of military engagements between Romanian and Austro-Hungarian forces during October 1916, in World War I. The attempt of the Austro-Hungarian 1st Army to break through the Eastern Carpathians was simultaneous with that of the German 9th Army to force the passes of the Southern Carpathians. Both efforts failed.
The Second Battle of Petrozsény was a World War I military engagement between Romanian forces on one side and Central Powers forces on the other side. It was part of the wider Battle of Transylvania and resulted in a Romanian victory.
The Battle of Nagybár was a military engagement between Romanian forces on one side and Central Powers forces on the other side. It was part of the 1916 Battle of Transylvania during the Romanian Campaign of World War I. The battle resulted in a Romanian victory.
The Battle of Nagyszeben was a World War I military engagement fought between the forces of the Central Powers on one side and the forces of Romania on the other side. It was the decisive engagement during the Battle of Transylvania, and also the largest, involving four armies out of the five fighting in the region: two Romanian, one German, and one Austro-Hungarian.
Romania 
Austria-Hungary (2 October):
