The first period of the war was thus favorable to the Habsburg and Catholic causes. Between 1618 and 1620, revolt had been suppressed in Bohemia and an influential Rhenish electorate had been transferred from Calvinist to Catholic hands. Now, however, the northern Protestant princes took alarm. If they had viewed with composure the failure of Frederick's foolhardy efforts in Bohemia, they beheld with downright dismay the expansion of Bavaria and the destruction of a balance of power long maintained between Catholic and Protestant Germany. And so long as the ill-disciplined remnants of Frederick's armies were behaving like highwaymen, pillaging and burning throughout the Germanics, the emperor declined to consider the grant of any concessions. [Sidenote: 2. Danish Intervention. Christian IV] At this critical juncture, while the Protestant princes were wavering between obedience and rebellion, Christian IV of Denmark intervened and precipitated the second period of the war. Christian IV (1588-1648) was impulsive and ambitious: as duke of Holstein he was a member of the Holy Roman Empire and opposed to Habsburg domination; as king of Denmark and Norway he was anxious to extend his influence over the North Sea ports; and as a Lutheran, he sought to champion the rights of his German co-religionists and to help them retain the rich lands which they had appropriated from the Catholic Church. In 1625, therefore, Christian invaded Germany, supported by liberal grants of money from England and by the troops of many of the German princes, both Calvinist and Lutheran. [Sidenote: Wallenstein] Against the Danish invasion, Tilly unaided might have had difficulty to stand, but fortune seemed to have raised up a codefender of the imperialist cause in the person of an extraordinary adventurer, Wallenstein. This man had enriched himself enormously out of the recently confiscated estates of rebellious Bohemians, and now, in order to benefit himself still further, he secured permission from the Emperor Ferdinand II to raise an independent army of his own to restore order in the empire and to expel the Danes. By liberal promises of pay and plunder, the soldier of fortune soon recruited an army of some 50,000 men, and what a motley collection it was! Italian, Swiss, Spaniard, German, Pole, Englishman, and Scot,--Protestant was welcomed as heartily as Catholic,--any one who loved adventure or hoped for gain, all united by the single tie of loyalty and devotion to Wallenstein. The force was whipped into shape by the undoubted genius of its commander and at once became an effective machine of war. Yet the perpetual plundering of the land, on which it lived, was a constant source of reproach to the army of Wallenstein.