Already the two great rivals were bidding against each other for allies, and Charles, though his resources were less concentrated than those of Francis, could promise most. Leo X. for his own territorial ambition, and in fear of Luther, rallied to the side of the Emperor, the German princes seconded their suzerain, and the great struggle for the supremacy of Christendom began in March 1521. England by treaty was bound to assist France, but this did not suit Wolsey or Henry in their new mood, and the Cardinal pressed his arbitration on the combatants. Francis reluctantly consented to negotiate; but minds were aflame with a subject that added fierceness to the political rivalry between Charles and Francis. The young Emperor, when he had met the German princes at Worms (April 1521), had thrown down the gage to Luther, and thenceforward it was war to the knife between the old faith and the new spirit. Henry, we may be certain to the delight of Katharine, violently attacked Luther in his famous book, and was flattered by the fulsome praises of the Pope and the Emperor. In the circumstances Wolsey's voyage to Calais for the furtherance of arbitration was turned into one to conclude an armed alliance with Charles and the Pope. The Cardinal, who had bent all others to his will, was himself bent by the Emperor; and the arbitrator between two monarchs became the servant of one. By the treaty signed at Bruges by Wolsey for Henry, Charles contracted an engagement to marry his little cousin, Princess Mary, and to visit England for a formal betrothal in the following year.