Romania emerged from this campaign with a slight net territorial gain and won its most important battle during the First World War.

[2] Although largely overrun in the winter of 1916, Romania had managed to remain in the war, holding a territorial rump in Moldavia with assistance from Russian forces.

The 1,500-strong French military mission, headed by General Henri Berthelot, included 300 officers who provided expert instruction in the use of new weapons and tactics, particularly the conduct of a war of position.

The Allied troops were faced by Austro-Hungarian and German forces under the command of General Friedrich von Gerok (21 infantry battalions, 36 cavalry squadrons and 142 artillery pieces, of which 6 were heavy).

On the first day of the offensive, the Romanians broke the front on a length of 19 kilometres (12 miles), inflicting heavy losses in men, weapons and materiel.

West of the Siret, the German 9th Army had 12 divisions with 102 infantry battalions, 10 cavalry squadrons and 213 artillery batteries (31 heavy).

The German offensive began on the night of 5-6 August, with a violent 8 hours-long artillery bombardment which included poison gas shells.

Striking between the Siret and the Focșani-Mărășești railroad, the Germans advanced 3 to 10 kilometres (1.9 to 6.2 miles) northwards against the left flank of the Russian 4th Army.

The intervention of the 5th Romanian Division in this threatened sector stabilized the situation, shelling the left flank of the Germans as they attempted to cross the river along with the Russians.

The commander of the Russian 4th Army decided to delay the counterattack, allowing the Germans to advance towards Mărășești and threaten the rear of the Romanian 9th Division.

On the morning of 14 August, after a powerful artillery bombardment with high explosive and gas shells, the Germans attacked the Russian troops at Panciu and pushed them back, threatening the left flank of the Romanian 2nd Army at Oituz.

The attempt to seize Mărășești, at the left flank of the Romanian 1st Army (commanded by General Eremia Grigorescu since 12 August) failed.

On the next day, German troops advancing southeast of Panciu were halted by units of the Romanian 1st Army, supported by accurate Romanian-Russian artillery fire.

German prisoners reported extremely heavy casualties, stating that they "had not come across such stiff resistance since the battles of the Somme and Verdun".

On 28 August, Mackensen launched another attack, causing the Russians to leave the battlefield in large numbers after showing little resistance.

The infantry attack began at 10 am, and the Central Powers managed to push the Romanian line back 700 to 800 metres (770 to 870 yd).

The Gerok Group continued attacking in the ensuing days, pushing the Romanians back, driving a 6 kilometres (3.7 miles) wedge between the IV Corps' two divisions.

The Romanian IV Corps, reinforced with cavalry, mountain infantry and air support, attacked on 11 August and managed to recover some important heights.

On the afternoon of the 12th, the Romanians and Russians attacked Cireșoaia, the hill overlooking the town of Târgu Ocna, with support from the artillery of the 7th Divisions.

Around 1 September, the fighting at Mărășești and Oituz became one single front, stretching around 120 kilometres (75 miles) from the Siret River to Doftana.