The World War I, which broke out in the summer of 1914, regenerates in the ruling liberal coalition and monarchial institution hopes for the national union. The state and the army, however, are not ready for new war. There is no unanimity among the political and military circles regarding taking a side in the conflict. Because of its location and capable army, Bulgaria is a wanted ally both for the Central Forces (Germany, Austria- Hungary and Turkey), and for the Entente (Russia, France and Great Britain). The temporary successful actions of the Central Forces, the failure of the Entente’s Dardanelles’ operation and the loan of 500 million levs, given to Bulgaria, predetermine pro- German orientation of the ruling circles.

On August 24th, 1915 Bulgaria signs a military convention with Germany and Austria-Hungary and also an agreement with Turkey, and on September 10th , 1915 is annunciated a total mobilization. The strength of the Bulgarian Operating Army amounts 616 680 warriors and 960 cannons, grouped in three armies.

On October 1st O.S./14th N.S., 1915 Bulgaria declares war on Serbia. The Operating Army is subordinated to the Commander of the Allied Forces Field-Marshal August von Mackensen. Its objective is to take part in the crushing of Serbian Army and to secure the connection between the allies and Turkey.

During the maneuver period of war (1915-1916) the Bulgarian troops take part in two campaigns. The campaign in Serbia and Macedonia achieves defeat of the Serbian Army and throwing back Entente’s troops beyond the Greek border. The campaign in Dobrudzha and Romania finishes with defeating of the Russian- Romanian troops and setting the North Front up at the Danube River delta and along Seret River.

During the positional period of the war (1917-1918) the Bulgarian Army wages defensive actions, defending its positions both at the North and South Fronts.

Waging the campaign in Serbia and Macedonia, the Bulgarian Army carries out three operations.

“Morava” offensive operation (1st - 27th October of 1915) is part of the strategic operation of the Group of armies “Mackensen”, aiming defeat of the Serbian Army, and is under command of Lieutenant- General Kliment Boyadzhiev. In the course of operation Bulgarian troops take over the fortified sections of Pirot and Nish and set up control over the valley of Morava River.

”Ovche pole” offensive operation (October 1st - November 12th , 1915) is prepared by the Bulgarian High Command. It is carried out by the 2nd army under command of Lieutenant- General Georgi Todorov. In seven days the Bulgarian troops take over the towns of Kumanovo and Veles and enter the Vardar River valley. The northern operative group of the army drives back Serbian conterstrike at the front of Vranya, Gnilyane, Kachanik . The southern group stopes the advancing from Solun (Thessaloniki) along the valley of Vardar River British and French troops, and follows them to the Greek border.

”Kosovo” offensive operation (October 18th - November 21st, 1915) continues “Morava” operation. After forcing of Morava River , units of the 1st army throw back the enemy, take over Prokuple town and go in pursuit , which is finished with seizure of Prishtina (today Pristina in Serbia) and Kosovo Field. Units of the north group of the 2nd army, after repulsing Serbian counterstrike at Ferizovo and Gnilyane , cross over Shar Planina Mountain and enter Prizren town. On November 16th, part of the Serbian troops surrenders and the main forces abandon their heavy machines and armored vehicles, cross over the Albanian Mountains and reach the Adriatic seacoast.

Romania’s intervening in the war on the side of Entente (August 27th, 1916) openes second front on the Balkans. “Dobrudzha” operation (September 1st, 1916 – January 3rd ,1917) is part of the strategic operation of the group of armies “Mackensen”, aiming to take Romania out of the war. The operation is accomplished by 3rd Bulgarian army under command of General Stefan Toshev.

On September 6th, 1916, after three days of fight, by decisive assault, under command of Major-General Panteley Kiselov, parts of 4th Preslav Infantry division and 1st Sofia Infantry division take over the Tutrakan Fortress – the strongest point of the Romanian defense in South Dobrudzha.

On September 7th, near Dobrich city, Varna Detachment under command of Major-General Todor Kantardzhiev, strengthened by two regiments of 6th infantry division, one Turkish regiment, together with 1st cavalry division of Major-General Ivan Kolev, crushes one Russian and one Serbian division.

Suffered a defeat on the line Tutrakan-Dobrich and in the encounter battles of Oltina- Karaomer- Mangalia, the Russian- Romanian Army withdrows back into the strengthen line Rasovo-Kubadin-Tuzla. On October 21st, 3rd army makes a decisive break through the center of Kubadin position. After seizuring Kyustendzha and Cherna Voda (On Danube River) , persuit of the enemy suspends in the narrowest part of North Dobrudzha. Part of the Bulgarian units are in the strength of the Danubian Army which forces Danube River at Svishtov-Zimnich zone and after seizuring of Bucharest follows the Romanian Army to the east till Seret River. On December 15th , 3rd army resumes the offensive in North Dobrudzha , brakes through the strengthen position between Cherna village and Danube River, and on January 3rd , 1917 takes over Machin town – the last stronghold of the Russian troops south of Danube River.

During the positional period of the war (1917-1918) decisive importance attaines the South front. From Shkumba River (to the west of Ohrid Lake) till the mouth of Struma River, the front with longitude of 354 km, is defended by 11th German Army, composed mainly of Bulgarian infantry units of 1st and 2nd Bulgarian armies. The Byalo More (Eng. - Aegean Sea) coast between Struma River and Maritsa River is defended by 4th Bulgarian army, which is formed in November 1917. To September 1918 the total strength of the Bulgarian Operating Army reaches up to 697 157 men and 1 459 cannons, and the total number of the mobilized Bulgarians increased to 857 063 men.

Overstretched on a vast front, without operative and strategic reserves, poorly supplied, the Bulgarian army almost three years holds the pressure of the growing up Entente troops. Memorable are the combats at Chervenata Stena, Zavoya na Cherna River , Dobro Pole, Yarebichna peak and Doyran.

On September 15th, 1918 starts powerful Entente offensive on the front line from Ohrid Lake to the town of Doyran. 9th infantry division under command of Major-General Vladimir Vazov, defending the Doyran section, withstands two- days’ artillery attack and 14 toxic gas attacks. In a fierce battle, which lasts 4 days and nights, it defeats two Britain and one Greek divisions. The losses of the enemy are 11 673 dead and 547 prisoners of war, and the losses of 9th division – 1728 dead and missing and 988 wounded warriors.

In the direction of the main blow the Entente Command conglomerates almost fourfold superiority, and on September 17th , 1918 the enemy breaks the front through the Bulgarian positions at Dobro Pole. Discontent among the retreating units turns into soldier’s uprising against the government and monarchy. On September 29th, 1918, on demand of the Bulgarian government is signes the Armistice of Solun (Thessaloniki). The Bulgarian troops to the west of Skopie meridian remain as hostages, but nevertheless, not a single Bulgarian combat flag is taken by the enemy.

The participation of Bulgaria in the World War I is the heaviest military effort in the Bulgarian history, it have led to excessive mobilization tension and to exhaustion of the country’s economic resources. The military operations of great intensity and continuity cost to Bulgaria 101 224 dead and missing and 155 026 wounded.