May 26 1921, Oppeln [Góra Świętej Anny]–The Polish and French had attempted to prevent Freikorps from reaching Upper Silesia, but were only able to delay them by a few weeks.  On May 21, the Germans launched an attack on the Annaberg hill, which dominated the surrounding Oder valley.  They were able to take the hill and hold it against Polish counterattacks, but a lack of artillery prevented them from pushing much beyond it.  The Poles officially abandoned their efforts to retake the hill on May 26, and fighting only continued on a limited basis for the next month.  The final division of Upper Silesia would largely follow the German-Polish lines after the battle.

Poles Seize Portions of Silesia

Polish paramilitaries with a derailed train during the uprising.

May 3 1921, Katowice–The Upper Silesian plebiscite in March had retuned a solid majority for Germany over Poland. Unlike most other post-war plebiscites, however, the plebiscite results did not immediately determine the border, but would instead be used by the Allies to guide the drawing the border. This left a lot of room for interpretation, with the French wanting to assign most of the highly-industrialized parts of Silesia to their Polish allies, but the British wanting them assigned to Germany to make sure the German economy could afford to pay war reparations to Britain.

As April progressed, the Poles feared that the British position would prevail at the negotiations, and they began to plan to force a fait accompli on the ground. On the night of May 2-3, Polish special forces destroyed all the rail connections between the area and the rest of Germany, and the next day Polish paramilitaries moved in and seized control of much of the disputed area. There were Allied troops in the area, but French forces were largely content to give the Poles free rein, while the British and Italians could only offer limited support to the Germans; no concerted effort was made to stop the fighting or get the Poles to withdraw

The destruction of the rail bridges, and a French decree against the arrival of paramilitaries from the rest of Germany, made it difficult for German reinforcements to arrive (or for reprisals to be carried out against the local Polish population), but Freikorpsmembers trickled into the area nonetheless over the next few weeks.

Upper Silesia, Poland by Arkadiusz Dziubany