The idea of a Bulgarian state including most of Macedonia, Thrace and Moesia was suggested under the Treaty of san Stefano, signed after the end of the Russian-Ottoman war of 1878. A russophile Bulgarian state would act as an Anchor for Russian influence on the region.
Russian efforts to exert influence on the area didn’t go unnoticed, though. All the bulgarian achievements were reversed by the following Treaty of Berlin, on which Bulgaria lost a large amount of its territory and lost its sovereignty, being reduced to a mere ‘principality’ status.
After the First balkan war, Bulgaria, along with the Balkan league (Greece, Serbia and Montenegro) fought against the Ottoman empire. Bulgaria gained control over the eastern part of Macedonia, and Thrace. However, the Bulgarian government was not satisfied with these gains, because it also wanted to annex the remaining part of Macedonia under Serbian control and the city of Thessaloniki (Solun), recently liberated by Greece. Bulgaria, despte having the strongest army on the Balkans (350.000 of the 700.000 troops were Bulgarian, and also the cuntry was nicknamed “The Prussia of the Balkans”), couldn’t withstand an attack from all sides, especially after the declaration of war issued by the Ottomans. Bulgaria lost control over Macedonia and East Thrace.
A new attempt to expand its territory came in WW1. Bulgaria, on the side of the central powers, fought against Greece, Serbia and Romania. They planned to finally conquer the lost territoriy of Macedonia, Dobruja, along with a large chunk of Serbian territory. Bulgaria, fighting on the wrong side, lost the war and all the occupied territories, plus West Thrace (Bulgaria couldn’t access the Aegean sea and the Mediterranean without passing through Turkish-dominated Bosphorous) and Southern Dobruja.
Another opportunity to expand its territory came with WW2. Starting with the treaty of Craiova, where bulgaria recovered Southern Dobruja and culminated with the Invasion of Yugoslavia and Greece, where Bulgarian soldiers once again Occupied Thrace, Macedonia and parts of Serbia. With the Invasion by the Soviet union in 1944, Bulgaria lost the war and all the occupied lands. The end of the war in 1945 and the Paris peace Conference marked the last effort by Bulgaria to conquer those lands. Southern Dobruja was kept, though.
Territories usually claimed by the Bulgarian Irredentists over the course of the last century are marked in shades of red. Bulgarian gains in WW1 also included the eastern half of Serbia