The Battle of (the) Annaberg (Polish: Bitwa o Górę Św. Anny) was the biggest battle of the Silesian Uprisings. The battle, which took place between May 21–26, 1921, was fought at the Annaberg (Polish: Góra Św. Anny), a strategic hill near the village of Annaberg O.S. (Góra Świętej Anny), located southeast of Oppeln (Opole) in Upper Silesia, Weimar Germany. After the hill had been captured by irregular Polish-Silesian units in the Third Silesian Uprising, German Freikorps pushed the Polish forces back. The final border was determined by political and diplomatic efforts.

By the Treaty of Versailles, the German Reichswehr was limited to a strength of 100,000. Several independent paramilitary Freikorps units were formed from the remnants of the German Imperial Army. The German Freikorps units often did not obey orders from the official government, but the German government assisted in transportation and supplies. Freikorps units fought against the communist groups in Germany and also against Polish insurgents in the East. While Germany had recognized the independent Polish state in the aftermath of Versailles, there were some disputed areas, some of which saw violent conflict.

On April 30, 1921, Polish-Silesian officials led by Wojciech Korfanty, upon finding that Germany would be granted most of the plebiscite area in disputed Upper Silesia, decided to start the Third Uprising even though the government in Warsaw wanted to avoid hostilities at all cost.

On May 2, acts of sabotage by the Polish Wawelberg Group units under Konrad Wawelberg severed all connections between Upper Silesia and Germany.

On May 3, at 3 a.m., the Polish forces started an offensive and in the following days they pushed the small German forces westwards, reaching the line of the Oder River and capturing the 400 m strategic hill of Annaberg on May 4.

It took around two weeks for the Germans to prepare the counteroffensive and to bring in volunteers from other German areas. The leaders settled for Generalleutnant Karl Höfer as commander. Generalleutnant Bernhard von Hülsen would lead the southern force at the Oder, and Oberstleutnant Grüntzen would lead the northern force in the forests.

The German units were strengthened by the arrival of the Freikorps Oberland unit from Bavaria. Its 1650 soldiers were experienced veterans of World War I, under Major Albert Ritter von Beckh. Among members of the FK Oberland were notable figures of the future Nazi Germany, including Sepp Dietrich (who distinguished himself during the battle), Rudolf Höss, Kurt Eggers, Edmund Heines, Beppo Römer, and Peter von Heydebreck, leader of the Werewolves and later pronounced the "hero of Annaberg". Also, there were several student-volunteers from the Bavarian town of Erlangen. The German force also consisted of Silesian paramilitary battalions (Selbstschutzes Oberschlesien), consisting of recently demobilized veterans and men too young to have fought in World War I.

Although the Polish forces outnumbered the German troops in the region, the Germans had more experience than the Poles, many of whom were civilians.