Gustavus now made a triumphal march, loved and respected by both Catholics and Protestants, through the garden spot of Germany. After resting his weary troops in the rich district of Wurzburg, he continued his march to Frankfort-on-the-Main, which opened her gates at the first summons. Gustavus crossed the Rhine at Oppenheim and on December 20th he entered Mainz, having cleared the Palatinate of its Spanish garrison. The Elector of Saxony and General Armin invaded Bohemia and occupied Prague. Gustavus now commanded from the Arctic Ocean to the Alps, and from the Rhine to the Moldau. He encamped at Main with an army of twenty thousand men. All the Protestant princes here paid him court. It was his plan to form a Corpus Evangelicum, or Union, under the protection of Sweden, which would include what is now called Germany and all Scandinavia.
France was much disconcerted by these Swedish victories. It was all right to curtail the pretensions of the House of Hapsburg, but to the eyes of Richelieu it would be a much worse fate for a new Emperor of Germany to bear the name of Gustavus than to let the Hapsburgs have undue sway.
Richelieu now insisted that Gustavus must come to an understanding with the Catholic League of southern Germany. Gustavus refused to surrender his conquests till the League saw that Maximilian of Bavaria, who was the head of the League, was disarmed. Richelieu at that broke his alliance with Gustavus and renewed his alliance with the Emperor. This was quite equivalent to a declaration of war. Richelieu declared himself the protector of the Catholic princes. They again took heart and brought together their armies in behalf of Austria.
The Jesuits who were at the Protestant courts succeeded in again stirring up the envy and jealousy of those weak northern Protestant German Electors. The Elector of Saxony now went back in heart, if not in force, to Austria.
Gustavus had felt that this would occur, and this was the main reason he had not pushed his triumph to Vienna. He now quickly conquered Franconia. Frankfort-on-the-Main, instead of opening her gates in welcome at his approach, wanted a parley. This city had received special commercial advantages from the Empire, and now they feared if Gustavus were well received they would lose their celebrated fairs. When summoned to surrender they sent a deputation to the King of Sweden explaining these conditions and hoping he would not urge compliance with his demands.
Gustavus was justly indignant. He said: "I am very much astonished that when the liberties of Germany and the Protestant religion are at stake the citizens of Frankfort talk of annual fairs, and postpone for temporal interests the great cause of their country and their conscience." He continued: "I have managed to find the keys of every town and fortress from the Island of Rugen to the Main, and I know where to find the key of Frankfort. The safety of Germany and the freedom of the Protestant faith are the sole objects of my invasion. Conscious of the justice of the cause, I am determined not to allow any object to impede my progress. I am well aware that the inhabitants of Frankfort wish to stretch out only a finger to me, but I must have the whole hand to grasp."
With his army he escorted the deputies back to the city, and in full battle array awaited the decision of the city. The gates were immediately opened and the entire army marched through the old imperial city, making a magnificent procession conducted in wonderful order. Here again the Protestant princes came to offer congratulations, to secure favors, or to appease his indignation at their heavy apathy. It was in Frankfort that the crown was yet voted upon, and placed on the one selected as head of the Holy Roman Empire.
Queen Eleanor Marie here visited him in company with Chancellor Oxenstiern. Neither of them approved of the brilliant court surrounding Gustavus, and the queen, with the swift intuitive knowledge which God gives to good women, felt that underneath all these protestations these Protestant princes had envy and jealousy in their hearts. Oxenstiern, that keen judge of men, came to the same conclusions. The king felt these things himself, and felt, also, that these princes, so divided among themselves, had little religion worth defending. He occasionally broke out in public, showing his surprise and pain at the attitude of their minds. On one occasion he said: "I wish to make peace if I am offered honorable conditions, such as will secure the welfare of the Protestant princes and their oppressed subjects, for whose sake I have undertaken this war and shed my blood. But I shall never conclude a peace by which the honor of Protestant princes would be sacrificed, their unhappy subjects bear an iron yoke, and our religion compromised."
George, Landgrave of Hesse Darmstadt, professed great friendship for the king, but secretly kept up a continued intercourse with the Emperor. On one occasion, when an unusual number of traitors happened to be among his guests, Gustavus said to George of Hesse Darmstadt: "If the Emperor does not care for me I shall not care for him. You may inform him of this, for I know you are well disposed towards his majesty." The landgrave was greatly confused by this unexpected thrust and stammered some excuse, but Gustavus continued: "He who receives $30,000.00 a year has indeed a reason to be the Emperor's friend. Were I to make such a present to anyone he must have well deserved it. It would be easy for me to enter into negotiations did I not consider the danger to those who have assisted me in this war for the restoration of the true religion."
That which the friends of Gustavus most feared was assassination. He suddenly awoke one night and found an armed man who proved to be a Catholic priest of Antwerp in his room. Jesuits were sent through the country to circulate calumnies against him. In Menz public prayer was offered for him, and at the same time an assassin was paid to take his life.