Ferdinand is in a bad financial situation in 1625, despite the subsidies received from Spain and the Pope.

In order to muster an imperial army to continue the war, he has applied to Wallenstein, now one of the richest men in Bohemia: the latter accepts on condition that he can keep total control over the direction of the war, as well as over the booty taken during the operations.

In order to aid the emperor against the Northern Protestants and to produce a balance in the Army of the Catholic League under Johann Tserclaes, Count of Tilly, Wallenstein has offered to raise a whole army for the imperial service following the bellum se ipsum alet principle.

The Latin phrase bellum se ipsum alet or bellum se ipsum alit (English: War feeds itself, French: La guerre doit se nourrir elle-même), and its German rendering Der Krieg ernährt den Krieg, describe the military strategy of feeding and funding armies primarily with the potentials of occupied territories.

The phrase, coined by the ancient Roman statesman Cato the Elder, is primarily associated with the Thirty Years' War.

Appointed imperial generalissimo on April 7, 1625, Wallenstein, viewing the war as a business enterprise, has raised a huge new imperial army at his own expense; he will take his profits in the form of levies imposed on conquered territories.

After receiving his final commission on July 25, 1625, his popularity soon recruits thirty thousand (not long afterwards fifty thousand) men.

The growing power of the Catholics in North Germany in and after 1623 is a threat to the Danish holdings in the Schleswig-Holstein duchies and has nearly induced Christian IV of Denmark to intervene directly in the Thirty Years' War.

Although he has stayed his hand for a time, the urgent solicitations of the western powers, and his fear that Gustavus Adolphus of Sweden will supplant him as the champion of the Protestant cause, finally leads him on May 9, 1625 to plunge into war.

He also fears that Sweden could use a war to further expand their holdings in the Baltic Sea.

Christian embarks on a military campaign which is later to be known in Denmark and Norway as "The Emperor War" (Danish: Kejserkrigen, Norwegian: Keiserkrigen).

Ernst Casimir I of Nassau-Dietz (Dillenburg, December 22, 1573 – Roermond, June 2, 1632) is the eleventh child of John VI, Count of Nassau-Dillenburg and Elisabeth of Leuchtenberg.

After the death of his father, the county of Nassau had been divided among his five living sons, Ernst Casimir following him as Count of Nassau-Dietz and Stadtholder of Friesland; he had been appointed Stadtholder of Groningen and Drenthe in August 1625.

Primarily known as an outstanding military leader, he has served under Maurice in the siege of the cities of Steenwijk and Oldenzaal, and Frederick Henry, Prince of Orange during the Siege of Groenlo (1627) and the Siege of 's-Hertogenbosch.

As Stadholder of Groningen he founds the Nieuweschans fortress in 1628.

Although he owns little in Friesland, he is popular there, and people grant his heir the right to rule after his death.

The portrait of his wife and children, painted in 1621, is unusual in its lack of modesty.

The Netherlands and England sign the Treaty of The Hague on December 9, 1625.

Based on the terms of the treaty, the signatories agree to provide economic aid to Christian IV of Denmark during his military campaigns in Germany.

The German military commander Ernst von Mansfeld has disbanded his army on the Rhine shortly after the Battle of Stadtlohn on August 6, 1623, which had ended the "Palatine Phase" of the Thirty Years' War.

Mansfeld had in about 1624 paid three visits to London, where he had been hailed as a hero by the populace, and at least one to Paris.

James I, being the father-in-law of Frederick V, Elector Palatine, is anxious to furnish him with men and money for the recovery of the Palatinate, but it had not been until January 1625 that Mansfeld and his army of "raw and poor rascals" had sailed from Dover to the Netherlands.

Christian IV of Denmark, having renewed the Thirty Years' War under his leadership, had reentered Germany later in the year to take part therein.

With the entrance of Denmark into the war, Protestant forces that have been dealt one defeat after another are suddenly infused with fresh hope as Denmark becomes the first major European nation to formally enter the war since the Austrian defeat in the early years.

With the new alliance comes ambitious plans involving Christian of Brunswick, who had lost all but two thousand o his fifteen thousand man army at Stadtlohn.

The projected campaign assigns Christian to assault the forces of Tilly in the Rhineland, and for Mansfeld to challenge Wallenstein in the Archbishopric of Magdeburg.

Mansfeld, moving first, begins his march towards Dessau in early spring, 1626, at the head of a Dutch-financed army.

Wallenstein learns of his movements though, and hurries his troops, some twenty thousand strong, to Dessau as well and establishes a bridgehead on the eastern side of the Elbe River.

The two sides meet soon after and, on April 25, Mansfeld begins the battle against the inexperienced Wallenstein.

With his infantry and artillery skilled to give the illusion that his army is smaller than the Catholics, Mansfeld attempts to use the sheer weight of the number of his men to push across the river.

This proves a great mistake as when the full extent of Wallenstein's forces is exposed, the assault quickly proves to be a deathtrap for Mansfeld.

Before he manages to retreat, Mansfeld's casualties number at some four thousand men, a third of his army.

This victory makes Ferdinand Germany’s virtual military master.

Christian of Brunswick now finds himself with ample financial backing under a plan that involves Mansfeld, Christian IV, King of Denmark, and himself, pushing from the United Provinces and from Denmark.

Ordered to advance on the Rhineland, he has undertaken this mission but quickly found himself checked by Tilly in Hesse, and opted this time to retreat rather than fight.

Ill from the outset of the campaign, he dies at Wolfenbüttel on June 16, 1626, at the age of twenty-six.

The Thirty Years' War had hitherto been confined to opposing factions of the Holy Roman Empire, but with the participation of Christian IV now extends to other European powers, though Christian, as Duke of Holstein, is not a complete foreigner.

As a Lutheran, Christian has allied with Mansfeld in a military campaign he plans to start in Thuringia in central Germany, and then take to its south.

His intention is to bring relief to German Protestants, who had been severely defeated a few weeks earlier in the Battle of Dessau Bridge.

Christian has at his disposal from nineteen thousand to twenty-five thousand men, and had at first gained some successes, but he has not thoroughly planned the advance against the combined forces of the Holy Roman Emperor and the Catholic League, as promises of military support from the Netherlands and England fail to materialize.

Tilly succeeds in drawing Christian's army to Lutter am Barenberge (in present-day Germany) and forcing it into open battle.

The imperial infantry breaks through the Danish line on three occasions but each time is repulsed by a cavalry counterattack.

However, eventually the Danish army is no longer able to maintain its ground and when its entire artillery falls into the hands of the enemy, panic sets in and the Danes retreat towards the town of Stade.

The Danish losses are approximately six thousand dead and twenty-five hundred prisoners.

The battle, an irreparable blow to Christian IV and Denmark, forces the Protestant German princes to sue for peace.

Ferdinand could end the war now and be satisfied with Imperial Catholic gains; instead, he will soon issue the Edict of Restitution, which will bring Sweden into the war.

Mansfeld had hastily retreated up the Oder River into Silesia, and quickly raised another army, with which he intends to attack the hereditary lands of the house of Austria.

Pursued by Wallenstein he presses forward towards Hungary, where he hopes to accomplish his purpose by the aid of Bethlem Gabor, prince of Transylvania.

But when Gabor changes his policy and makes peace with the emperor, Mansfeld is compelled to disband his troops.

Wallenstein sends eight thousand troops to assist Tilly, who is stationed near Brunswick, while Mansfeld sets out for the Dalmatian coast to offer his services to the Republic of Venice, but when he reaches Rakovica near Sarajevo, in Bosnia, he is taken ill, apparently with tuberculosis, and here he dies on November 29, 1626.

He is buried at Split.

Both Tilly and Wallenstein occupy the duchies of Schleswig and Holstein and the entire peninsula of Jutland in the late summer of 1627, advancing through the peninsula as far as Limfjord.

The Danish armies, in the months since Dessau and Lutter, have been forced to abandon their gains on imperial soil and parts of Denmark herself, and have retreated to the Danish isles while Wallenstein’s imperial army has subsequently occupied the North German plain.

The Duchy of Pomerania capitulates at Franzburg in November 1627.

The Baltic Sea remains under Danish control, however, due to the lack of an imperial navy.