The Macedonian Front was the war's overlooked theater. After Bulgaria entered the war on the Central Powers' side in October 1915, an Allied force based at Thessaloniki found itself in a long stalemate against Bulgarian and German troops along a line that ran from the Adriatic to the Aegean. By 1917 the front had been static for more than a year, with the Allies holding the lower ground and the Bulgarians holding the heights. The Second Conference of the Military Counsel of the Entente at Chantilly directed the commanders at Thessaloniki to attempt a breakthrough, in coordination with offensives planned in France. The objective was to drive north along the Vardar valley toward Skopje and Belgrade, which would force Bulgaria out of the war and unhinge Austria-Hungary. The Bulgarian command had spent a year improving its positions. By the spring of 1917 the heights at Doiran were among the most heavily fortified positions on any active front in the war.

In early 1917 the 2nd Bulgarian Thracian Infantry Division was rotated out of Doiran and replaced by the 9th Pleven Infantry Division under Colonel Vladimir Vazov, an experienced officer from a famous Bulgarian military family. Vazov had something most defenders on the Western Front would have envied: terrain that did the work for him. The Bulgarian positions were on rocky heights, with clear fields of fire down into the British forming-up grounds 800 to 1,500 meters below. He had 30,000 men, 147 guns, 35 mortars, and 130 machine guns. He divided the front into three zones, with two brigades and a regiment covering ground from the Vardar River to Lake Doiran, and ordered systematic preparation of the artillery. When British intelligence confirmed an Allied buildup, the Bulgarians replied with their own buildup. Vazov's pre-attack barrage on the British forming-up grounds fired roughly 10,000 shells in a single hour. By the time British infantry began to climb the slopes, the routes they had to cross were already smashed.

The British concentrated three divisions for the assault: the 22nd, 26th, and 60th, with more than 43,000 men, 160 guns, 110 mortars, and 440 machine guns. The first major attack went in on the night of 24 to 25 April. The infantry climbed in darkness toward the Bulgarian wire and were caught by the prepared barrage and the counterattack that followed. By 8 pm on the 25th they had retreated. The second major attack came on the night of 8 to 9 May. The same pattern: bayonet assault on prepared positions in the dark, Bulgarian artillery cutting up the supporting waves, counterattack rolling the survivors back down the slope. The Times correspondent at Salonika wrote that the British soldiers called the Boris position the valley of death, and the name stuck. Vazov was promoted to Major-General. The artillery duel continued into early May, but the British command had run out of unwounded infantry and called off offensive operations. The front then sat in silence for sixteen months.

The Macedonian Front did finally break, but not at Doiran. In September 1918 a French and Serbian force punched through the Bulgarian line at Dobro Pole, about 100 kilometers to the west, and Bulgaria sued for peace within weeks. The Third Battle of Doiran in mid-September 1918 was a coordinated Anglo-Greek attack designed to pin Bulgarian reserves while the main breakthrough happened elsewhere; the Pleven Division, now without Vazov but still in roughly the same positions, repulsed the British and Greek assaults the same way it had in 1917. The Doiran heights were never taken in combat. The Swedish heavy metal band Sabaton wrote a song called The Valley of Death for their 2022 album The War to End All Wars, drawing on the British nickname. The lake itself today straddles the border between Greece and North Macedonia. War cemeteries in both countries hold the men who could not finish climbing the hill. The names on the headstones are British, Greek, French, Bulgarian. They were trying to take the same ground.