regiments of infantry towards the Spree. But the position which he held
in Germany, made it necessary that he should not move forward without
securing his rear. In traversing a country where he was surrounded by
suspicious friends and dangerous enemies, and where a single premature
movement might cut off his communication with his own kingdom, the
utmost vigilance and caution were necessary. The Elector of Brandenburg
had already opened the fortress of Custrin to the flying Imperialists,
and closed the gates against their pursuers. If now Gustavus should
fail in his attack upon Tilly, the Elector might again open his
fortresses to the Imperialists, and the king, with an enemy both in
front and rear, would be irrecoverably lost. In order to prevent this
contingency, he demanded that the Elector should allow him to hold the
fortresses of Custrin and Spandau, till the siege of Magdeburg should be
raised.

Nothing could be more reasonable than this demand. The services which
Gustavus had lately rendered the Elector, by expelling the Imperialists
from Brandenburg, claimed his gratitude, while the past conduct of the
Swedes in Germany entitled them to confidence. But by the surrender of
his fortresses, the Elector would in some measure make the King of
Sweden master of his country; besides that, by such a step, he must at
once break with the Emperor, and expose his States to his future
vengeance. The Elector's struggle with himself was long and violent,
but pusillanimity and self-interest for awhile prevailed. Unmoved by
the fate of Magdeburg, cold in the cause of religion and the liberties
of Germany, he saw nothing but his own danger; and this anxiety was
greatly stimulated by his minister Von Schwartzenburgh, who was secretly
in the pay of Austria. In the mean time, the Swedish troops approached
Berlin, and the king took up his residence with the Elector. When he
witnessed the timorous hesitation of that prince, he could not restrain
his indignation: "My road is to Magdeburg," said he; "not for my own
advantage, but for that of the Protestant religion. If no one will
stand by me, I shall immediately retreat, conclude a peace with the
Emperor, and return to Stockholm. I am convinced that Ferdinand will
readily grant me whatever conditions I may require. But if Magdeburg is
once lost, and the Emperor relieved from all fear of me, then it is for
you to look to yourselves and the consequences." This timely threat, and
perhaps