The Struma operation or Battle of Struma was a successfull offensive operation of the Bulgarian army during the First World War between 17 - 23 August 1916 in which they conquered all Greek territory east of the Struma river.

In August 1916 Romania chose to join the war effort on the side of the Entente. The Allies planned a large offensive in the Macedonian Front for the middle of August in order to support Romania's entry in the war and pin down as many Bulgarian forces as possible. The Bulgarian high command suspected an impending offensive and the fighting around Doiran that erupted on 9 August only confirmed these suspicions. On their part the Bulgarians had urged for an offensive in Macedonia since the beginning of the year and now planned a strike with the First Army and Second Army on both Allied flanks.

The plan on the eastern flank was to seize the Drama - Giumurdzhina railway and this objective was given to the Bulgarian Second Army and the 10th Aegean Division. For the operation general Todorov could rely on 58 battalions, 116 machine guns, 57 artillery batteries and 5 cavalry squadrons in his army and an additional 25 battalions, 24 machine guns, 31 batteries and 5 squadrons in the 10th Division.

The Struma Offensive began on 18 August with the 7th Rila, 11th Macedonian Division and the 3/2 Infantry Brigade and 10th Division advancing on a 230 kilometer long front. For 6 days the Bulgarian forces achieved all their objectives in the face of weak Greek and French resistance. The depth of the advance reached 80-90 kilometers and an area of 4,000 square kilometers was occupied. Most importantly however the Macedonian Front was shortened with 100 - 120 kilometers and the Greek IV Army Corps, numbering 464 officers and 6373 soldiers that was positioned in the area, was disbanded, its troops and armament were interned by the Germans in Silesia for the rest of the war.
The cities of Kavala, Drama and Serres were taken.

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v t e Bulgaria during World War I

Prelude South-western front Serbian campaign, Macedonian front Romanian front • Outcome • Others Important persons Balkan Wars 1912–1913 First Balkan War 1913 Treaty of London Second Balkan War Treaty of Bucharest Treaty of Constantinople Neutrality 1914 Ottoman–Bulgarian alliance 1915 Negotiations of Bulgaria with the Central Powers and the Entente Bulgarian–German treaty Secret Bulgarian–German agreement Military convention between Germany, Austria-Hungary and Bulgaria Bulgarian–Ottoman convention Mobilization Commanders  Bulgaria Nikola Zhekov • Kliment Boyadzhiev • Dimitar Geshov • Georgi Todorov • Ivan Lukov • Stefan Nerezov • Vladimir Vazov Entente:  Serbia: Radomir Putnik • Živojin Mišić • Stepa Stepanović • Petar Bojović • Pavle Jurišić Šturm;  France: Maurice Sarrail • Adolphe Guillaumat • Louis Franchet d'Espèrey;  United Kingdom: Bryan Mahon • George Milne;  Kingdom of Greece: Panagiotis Danglis Field Armies + Eleventh Army First Army Second Army Fourth Army Battles 1915 Morava Offensive • Ovče Pole Offensive • Kosovo offensive (1915) • Battle of Krivolak 1916 First battle of Doiran • Battle of Florina (Lerin) • Struma operation • Monastir offensive 1917 Second battle of Doiran • 2nd Crna Bend • Second battle of Monastir 1918 Battle of Skra-di-Legen • Battle of Dobro Pole • Third battle of Doiran Commanders  Bulgaria Nikola Zhekov • Panteley Kiselov • Stefan Toshev • Todor Kantardzhiev • Ivan Kolev Entente:  Romania: Constantin Prezan • Alexandru Averescu;  Russia: Andrei Zayonchkovski • Vladimir Sakharov Field Armies Third Army Battles 1916 Battle of Turtucaia • Battle of Bazargic • First Cobadin • Flămânda Offensive • Second Cobadin • Battle of Bucharest Outcome 1918 Treaty of Brest-Litovsk • Armistice of Focșani • Treaty of Bucharest • Protocol of Berlin Outcome Armistice of Salonica Vladaya revolt Treaty of Neuilly-sur-Seine Others Bulgarian administration in Kosovo Anti-military propaganda Ferdinand Vasil Radoslavov Aleksandar Malinov Aleksandar Stamboliyski Aleksandar Protogerov August von Mackensen Boris Drangov Vladimir Vazov Georgi Todorov (general) Ivan Kolev Kliment Boyadzhiev Dimitar Geshov Ivan Lukov Konstantin Zhostov Krastyu Zlatarev Nikola Zhekov Panteley Kiselov Stefan Toshev Stefan Nerezov Todor Kantardzhiev