Whether or not the German-Polish invasion of Bohemia actually happened, the friendly relations between the Czechs and the Poles came to an end. Bohemia resumed its earlier alliance with the Lutici, which, in 990, resulted in a war with Mieszko, who was supported by Empress Theophanu. Duke Boleslav II was probably the first one to attack. As a result of the conflict Silesia was taken over by Poland. However, the annexation of Silesia possibly took place around 985, because during this year the major Piast strongholds in Wrocław, Opole and Głogów were already being built.

The issue of the incorporation of Lesser Poland is also not completely resolved. Possibly Mieszko took the region before 990, which is indicated by the vague remark of Thietmar, who wrote of a country taken by Mieszko from Boleslav. In light of this theory, the conquest of Lesser Poland could be a reason for the war, or its first stage. Many historians suggested that the Czech rule over Lesser Poland was only nominal and likely limited to the indirect control of Kraków and perhaps a few other important centers. This theory is based on the lack of archaeological discoveries, which would indicate major building investments undertaken by the Bohemian state.

After its incorporation, Lesser Poland supposedly became the part of the country assigned to Mieszko's oldest son, Bolesław, which is indirectly indicated in the chronicle of Thietmar.