As a result of the involvement of Romania into war, at 14/27 August, 1916, and of the 2nd Army’s offensive in Transylvania, on 19 August/1 September Bulgaria declared war to Romania. The Bulgarian units from the southern border of Romania launched the attack during the night of 18-19 August/31 August-1 September 1916. On 22 August/4 September the battle of Bazargic started, the Bulgarian-German-Turkish troops taking the town. On 24 August/6 September, the 3rd Romanian Army troops were defeated at Turtucaia. Silistra was evacuated to avoid encirclement. The forces of the 3rd Bulgarian Army reached the Rasova, Cobadin, Topraisar alignment, where they were stopped at 3/16 September.
At 2/15 September 1916 the war council held at Peris decided to stop the offensive in Transylvania and to annihilate the enemy group on the southern front. The Army Group “South” was created for this (commanded by the general Alexandru Averescu). It was composed by the 3rd army and by the Army of Dobruja. The 3rd Army consisted by the 10th, 16th, 18th, 21st and 22nd Infantry Divisions and by the 1st Cavalry Division. The Army of Dobruja was composed by 2nd, 5th, 9th, 12th, 15th and 19th Infantry Divisions, the 5th Cavalry Brigade, and the 47th Russian Corps, consisting of 61st and 115th Infantry Divisions the 3rd Cavalry Division. The forces of the Army Group “South” totalized 186 infantry battalions, 55 cavalry squadrons and 148 artillery batteries. The Bulgarian-German-Turkish group, commanded by the Field Marshal August von Mackensen, had available forces having the strength of 8-9 divisions (105 infantry battalions, 35 cavalry squadrons and 70 artillery batteries).
The plan of operations stipulated to force the crossing of Danube at Flamanda (Rahovo), in the area of the 3rd Army, with 5-6 divisions supposed to develop an offensive on the direction Flamanda, Acadinlar, Kurtbunar. In the same time the Army of Dobruja was planned to start an offensive on the direction Cobadin, Kurtbunar, succeeding this way the encirclement of the enemy forces at the south of Danube.
The operation preparing started immediately after the war council held at Peris and lasted until 18 September/1 October. A scaffold was built across Danube at Tabanul. Access roads were built toward the break zone. In addition to the materials required to build a bridge, 200 boats were prepared. A campaign airfield was built at Dadilov. The anti-aircraft defense was ensured by 2 AA batteries, and for defense against vessels 2 artilery batteries were installed at Gostinu, one of 150 mm.
At 18 September/1 October the forcing of Danube started, using the 10th Infantry Division (at 3:30 hours) and the 21st Infantry Division (at 10 hours). At 5 hours the construction of the bridge started. Before sunset the 2 divisions made a bridgehead 4 km deep and 14 km wide.
In the night of 18-19 September/1-2 October, a strong storm broke several times the bridge, and the increase of the level of Danube allowed to several enemy vessels (“Barsch”, ”Viza”, “Bodrog”, “Koros”) to pass over scaffold and bombard the bridge, but they did not damage it. The Romanian artillery answered with fire and damaged 2 of the monitors. The enemy vessels withdrew to the Persina channel. The storm also flooded all the forcing area, with disastrous effects for the future of the operation.
In the same time with the forcing of Danube at Flamanda, the forces of the Army of Dobruja started the offensive. The results were below expectations, and until 22 September/5 October only advances between 3 and 8 km were succeeded through the enemy disposition.
The flooding of the Danube in the forcing sector and the weak progress of the Army of Dobruja determined the General Averescu to ask the canceling of the operation, and the withdrawal of the Romanian troops to the north of Danube. The withdrawal started in the night of 20-21 September/3-4 October, and was finalized during the day of 21st/4th.
In spite of the ingenuity of the plan, and the good preparation of the operation, the failure of the maneuver at Flamanda had grave consequences for the further development of the war, in special because it led to the non-observance of the campaign schedule, by changing the objective and the main effort from one front to the other.