On 17 September 1912 Tsar Ferdinand signed a special decree ordering the mobilization of the Bulgarian armed forces, and in accordance with the constitution of the country assumed the nominal role of commander-in-chief.

The chief of staff of the Bulgarian Army, Major General Ivan Fichev, had devised the final plan for the war with the Ottoman Empire more than a year earlier.

The commander of the Ottoman Army Kölemen Abdullah Pasha, under the pressure of his superiors, planned to attack the Bulgarians on their flanks and envelop them with his III Corps and the forces of the Adrianople fortress.

Almost immediately after the end of the First Balkan War, Bulgaria began to transfer its forces from Eastern Thrace to Macedonia in order to protect its threatened interests in the area.

By that time, the Bulgarian command finally decided to utilize the inactive 1st and 3rd Armies and gave them orders to take the Serbian town of Knjaževac and then with their combined force to seize the fortress of Pirot.

The bad weather, poor roads and fog hampered the speed of movement of the units and the strong Serbian positions prevented any important Bulgarian gains.

The Serbian command was concerned with these developments and decided to reinforce its troop with units from Macedonia, which weakened their advance there and allowed the Bulgarian 4th Army to contain it.

The Left Group of the 3rd Army however was ordered to retreat back to the border because it was considered needless to expose it to battle with the fresh Serbian reinforcements.

The advance of the Right Group began on 24 June; after a short struggle the Serbian vanguards were pushed back towards Pirot and the Bulgarians took up favorable positions on the high ground overlooking the field around the fortress from which their artillery could support any further attacks with great efficiency.

To compensate for this loss the command used the garrisons of Oryahovo, Gigen, Somovit, Nikopol and Svishtov as well as 2 March Brigade to form a new 12th Mixed Infantry Division.

On 14 October a Russian fleet that included the battleships Imperatritsa Mariya, Evstafi, Ioann Zlatoust, Panteleimon, the cruisers Pamiat Merkuria and Kagul supported by eighteen smaller vessels bombarded the port of Varna.

For the next several months no such landing materialized and in May 1916 the Bulgarian and German high commands agreed to send the 105th Division to Macedonia with the exception of a single infantry battalion, two machine guns and the 105th heavy battery that were to remain in Shumen and Varna.

Since the beginning of the First World War until the summer of 1916 the Central Powers had not considered any military response to a possible intervention of Romania on the side of the Allies and Austria-Hungary felt free to keep only small forces of soldiers and gendarmes in Transylvania.

On 28 July they agreed on several measures – not to engage in unnecessary provocations against Romania, in case the country joined the Allies then Bulgaria was obliged to undertake offensive actions by all means and Austria-Hungary as far as the circumstances allowed.

The appointment received the sanction of the Bulgarian commander-in-chief, General Zhekov, on 28 August and two days later the headquarters of Army Group Mackensen were established in Veliko Turnovo.

In addition, the Russian XLVII Corps with up to 40,000 men under the command of General Andrei Zayonchkovski had crossed the Danube and was slowly moving towards the Dobrudja border.

On the right Bulgarian flank the 6th Bdin Division and forces of the Varna Fortified Area formed up in a mobile reserve also advanced, meeting weak resistance in the process.

The Romanians could initially oppose the attackers with 19 battalions from the 15th division but they were quickly reinforced to 36 when the fighting began and could count on the support of 246 artillery pieces and 78 machine guns.

When the Battle of Tutrakan ended on 6 September two Romanian infantry divisions were practically destroyed – some 28,500 soldiers and officers along with over 100 cannons, 62 machine guns and thousands of rifles were captured by the Bulgarian Third Army.

During the final assault on Tutrkan the right wing of the Bulgarian Army came under attack from the Russian XLVII Corps and the Romanian 19th Division at the Battle of Dobrich.

After two days of heavy fighting the Bulgarians and Germans failed to break through and General Toshev decided to call off the attack as the exhaustion of the troops could make them vulnerable to counterattacks.

The Romanian operation soon stalled in the face of the bad weather, the determined resistance of the outnumbered defenders and the constant attacks of the Austrian Danube Flotilla on the bridge.

The entire center and right of the Russo-Romanian Army was retreating towards Cernavodă and Medgidia while the defensive line between the Danube and Cobadin had fallen in the hands of the Bulgarians and the Ottomans.

The remainder of the forces in Dobrudja were returned to General Toshev's command and ordered to dig in on a new defensive line between Lake Taşaul and the village of Boasgic (today Dunărea).

A cause for great concern was also the large Russian forces that were concentrating on the left bank of the Danube which prompted the general to ask the Ottoman VI Corps, that had remained directly under the army group control after Cobadin, to be placed under his command.

By the middle of November, the battle along the entire line was raging with full force but the misunderstandings and conflicts between Mackensen and the Third Army's command were also peaking.

Under German pressure the Bulgarian commander-in-chief, General Nikola Zhekov, relieved Toshev of his command and send him to Macedonia, where he initially served as governor.

On 1 and 2 December, the Russians made a final attempt to break through but were again repulsed losing considerable numbers of men and material including two armored cars that the Bulgarians captured.

For the remainder of the war, the Third Army remained in Dobrudja but after the capitulation of Romania and the Russian Empire its main units were transferred to the Macedonian front leaving behind only some garrison and occupation forces.

On 21 September in accordance with the Treaty of Craiova the army entered Southern Dobrudja that was returned to the Kingdom of Bulgaria after 26 years of Romanian rule.