The king had scarcely spent more time in conquering Franconia than he would have required to cross it. He now left behind him Gustavus Horn, one of his best generals, with a force of 8,000 men, to complete and retain his conquest. He himself with his main army, reinforced by the late recruits, hastened toward the Rhine in order to secure this frontier of the empire from the Spaniards, to disarm the ecclesiastical electors, and to obtain from their fertile territories new resources for the prosecution of the war. Following the course of the Main, he subjected, in the course of his march, Seligenstadt, Aschaffenburg, Steinheim, the whole territory on both sides of the river. The imperial garrisons seldom awaited his approach, and never attempted resistance. In the meanwhile one of his colonels had been fortunate enough to take by surprise the town and citadel of Hanau, for whose preservation Tilly had shown such anxiety. Eager to be free of the oppressive burden of the Imperialists, the Count of Hanau gladly placed himself under the milder yoke of the King of Sweden. Gustavus Adolphus now turned his whole attention to Frankfort, for it was his constant maxim to cover his rear by the friendship and possession of the more important towns. Frankfort was among the free cities which, even from Saxony, he had endeavored to prepare for his reception; and he now called upon it, by a summons from Offenbach, to allow him free passage and to admit a Swedish garrison. Willingly would this city have dispensed with the necessity of choosing between the King of Sweden and the Emperor; for, whatever party they might embrace, the inhabitants had a like reason to fear for their privileges and trade. The Emperor's vengeance would certainly fall heavily upon them, if they were in a hurry to submit to the King of Sweden, and afterward he should prove unable to protect his adherents in Germany. But still more ruinous for them would be the displeasure of an irresistible conqueror, who, with a formidable army, was already before their gates, and who might punish their opposition by the ruin of their commerce and prosperity. In vain did their deputies plead the danger which menaced their fairs, their privileges, perhaps their constitution itself, if, by espousing the party of the Swedes, they were to incur the Emperor's displeasure. Gustavus Adolphus expressed to them his astonishment that, when the liberties of Germany and the Protestant religion were at stake, the citizens of Frankfort should talk of their annual fairs, and postpone for temporal interests the great cause of their country and their conscience. He had, he continued, in a menacing tone, found the keys of every town and fortress, from the Isle of Rugen to the Main, and knew also where to find a key to Frankfort; the safety of Germany, and the freedom of the Protestant Church, were, he assured them, the sole objects of his invasion; conscious of the justice of his cause, he was determined not