In spite of the urgent entreaties of Gustavus and the pressing peril of Magdeburg, the wavering Duke of Brandenburg could not bring himself to join the Swedes.

He delivered Spandau over to them, but would do no more.

The Swedish army accordingly marched to Berlin and invested his capital.

The duke sent his wife to Gustavus to beseech him to draw off his army and allow him to remain neutral; but Gustavus would not listen to his entreaties, and insisted, as the only condition upon which he would raise the siege, that the duke should ally himself with him, and that the troops of Brandenburg should join his army.
These conditions the duke was obliged to accept, but in the meantime his long hesitation and delay had caused the loss of Magdeburg, which after a gallant defence was stormed by the troops of Pappenheim and Tilly on the 10th of May.

The ferocious Tilly had determined upon a deed which would, he believed, frighten Germany into submission; he ordered that no quarter should be given, and for five days the city was handed over to the troops.
History has no record since the days of Attila of so frightful a massacre.