GERO (c. 900-965), margrave of the Saxon east mark, was entrusted in by the German king Otto, afterwards the emperor Otto the Great, with the defence of the eastern frontier of Saxony against the Wends and other Slavonic tribes. In a few years he extended the Saxon frontier almost to the Oder, and suppressed a rising of the conquered peoples in a great victory on Oct. 16, 955. In 963 he defeated the Lusatians and compelled the king of the Poles to recognize the supremacy of the German king.