Third Bulgarian Empire in the context of "Principality of Bulgaria"

The Eastern Front, also known as the Great Patriotic War in the Soviet Union and its successor states, and the German–Soviet War in modern Germany and Ukraine, was a theatre of World War II fought between the European Axis powers and Allies, including the Soviet Union (USSR) and Poland. It encompassed Central Europe, Eastern Europe, Northeast Europe (Baltics), and Southeast Europe (Balkans), and lasted from 22 June 1941 to 9 May 1945. Of the estimated 70–85 million deaths attributed to the war, around 30 million occurred on the Eastern Front, including 9 million children. The Eastern Front was decisive in determining the outcome in the European theatre of operations in World War II and is the main cause of the defeat of Nazi Germany and the Axis nations. Historian Geoffrey Roberts noted that "more than 80 percent of all combat during the Second World War took place on the Eastern Front".

The Axis forces, led by Germany, invaded the Soviet Union in Operation Barbarossa on 22 June 1941. Despite warnings and the deployment of Axis armies on his borders, Stalin refused to believe that Hitler would invade and forbade any defensive preparations. Thus the Soviets were caught completely unprepared. They were unable to halt deep Axis advances into Russia, which came close to seizing Moscow. However, the Axis failed to capture the city, and Hitler shifted his focus to the oil fields of the Caucasus the following year. German forces advanced into the Caucasus under Fall Blau ("Case Blue"), launched on 28 June 1942. The Soviets decisively defeated the Axis at the Battle of Stalingrad—the bloodiest battle in the war and arguably in all of history—making it one of the key turning points of the front. A second great Axis defeat, at the Battle of Kursk, crippled German offensive capabilities permanently and cleared the way for Soviet offensives. Several Axis allies defected to the Allies, such as Italy, Romania and Bulgaria. The Eastern Front concluded with the capture of Berlin, followed by the signing of the German Instrument of Surrender on 8 May, ending the Eastern Front and the war in Europe.

The Bulgarian Resistance (Bulgarian: Партизанско движение в България, romanized: Partizansko dvizhenie v Balgariya, lit. 'Partisan movement in Bulgaria') was part of the anti-Axis resistance during World War II. It consisted of armed and unarmed actions of resistance groups against the Wehrmacht forces in Bulgaria and the Tsardom of Bulgaria authorities. It was mainly communist and pro-Soviet Union. Participants in the armed resistance were called partizanin (a partisan) and yatak (a helper, or a supporter, someone who provides cover for someone else).

Saxe-Coburg and Gotha (German: Sachsen-Coburg und Gotha), or Saxe-Coburg-Gotha (German: Sachsen-Coburg-Gotha [ˈzaksn̩ ˈkoːbʊʁk ˈɡoːta]), was an Ernestine duchy in Thuringia ruled by a branch of the House of Wettin, consisting of territories in the present-day states of Thuringia and Bavaria in Germany. It lasted from 1826 to 1918. In November 1918, Charles Edward, Duke of Saxe-Coburg and Gotha, was forced to abdicate. In 1920, the northern part of the duchy (since 1918 the Free State of Gotha; culturally and linguistically Thuringian) was merged with six other Thuringian free states to form the Free State of Thuringia: Saxe-Weimar-Eisenach (until 1918 a grand duchy), Saxe-Altenburg and Saxe-Meiningen (until 1918 duchies), Schwarzburg-Rudolstadt and Schwarzburg-Sondershausen (until 1918 principalities), as well as the People's State of Reuss (until 1918 the principalities of Reuss-Gera and Reuss-Greiz). The southern part of the duchy (since 1918 the Free State of Coburg; culturally and linguistically Franconian), as southernmost of the Thuringian states, was the only one which, after a referendum, became part of the Free State of Bavaria.

The name Saxe-Coburg-Gotha also refers to the family of the ruling House of Saxe-Coburg and Gotha, which played many varied roles in the dynastic and political history of Europe in the 19th and 20th centuries. In the early part of the 20th century, before the First World War, it was the family of the sovereigns of the United Kingdom, Belgium, Portugal, Bulgaria, and Saxe-Coburg-Gotha. In 1910, the Portuguese king was deposed, and the same thing occurred in Saxe-Coburg-Gotha in 1918 and in Bulgaria in 1946 (although effective control had already ceased two years prior due to a coup). As of 2025, a branch of the family still reigns in Belgium, and while the British line still technically exists up to the present day, it now uses the name Windsor due to a proclamation issued by King George V in 1917. The former Tsar of Bulgaria, Simeon II (reigned 1943–46), kept his surname while serving as the Prime Minister of Bulgaria from 2001 to 2005.