On September 7, 1940, the treaty of Craiova was signed, which stated that Romania would give up Southern Dobruja. After this the pressure to join the Tripartite Pact increased.
Germany needed to go through Bulgaria to invade the Balkans, and as a result on March 1, 1941, Minister President Bogdan Filov signed an agreement to join the Tripartite Pact. The next day the German forces crossed the Danube on its way to invade Greece.
New Mechanized Forces
In June 1941 the 1st Armored Brigade was formed which was made up of the 1st Tank Regiment and 1st Mechanized Infantry Regiment. The tank regiment had a reconnaissance company of the CV 33 tankettes. The 2 tank battalions were made up of the PzKpfw 35(t)s and Vickers 6-ton in one, and 40 French R-35s donated by the Germans. The regiment never saw much action.
In July 1943, the Germans updated the regiment with 46 PzKpfw IVs, 10 PzKpfw IIIs, 25 StuG IIIs, and SdKfz 222, 223s.
On March 5th Britain severed diplomatic relations. It wasn't until December 13, 1941, that Bulgaria declared war on Britain and the United States. However, Bulgaria never declared war against Russia.
King Boris tried to insist that his army wasn’t mechanized and couldn't fight well outside of the Balkans. He tried to persuade the Germans that they would act as a buffer against the Turks and any Balkan invasion by the Allies.
Bulgarian forces did invade Yugoslavia and administered large parts of Yugoslav Macedonia. After Greece was conquered, Bulgaria administered eastern Macedonia and western Thrace.
Bulgaria was constantly pressured to help the Germans in Russia, but the king always refused. In the summer of 1943 Bulgaria refused to fight the partisans any further in Yugoslavia and Albania.
The King Dies
The king died on August 28, 1943, and a regency took over. Filov was replaced by Dobri Bozhilov on September 14, 1943. He started to try and negotiate with the Allies.