The real conflict, which lasted almost 80 years and spanned three generations, escalated in the 1620s, when the line of Hesse-Marburg died out, and peaked after 1645 in the Hessian War proper.

[5] During the Thirty Years' War, Calvinist Hesse-Cassel joined the alliance between Protestant Sweden and Catholic France, whilst despite its Lutheran beliefs, Hesse-Darmstadt, sided with the Emperor.

This conflict should not be confused with the campaign by Landgrave Philip I of Hesse, supported by Prince-Elector John of Saxony against the two Franconian prince-bishoprics of Würzburg and Bamberg in 1528, which is also referred to as the "Hessian War".

Darmstadt would have benefited from the latter interpretation, because George of Hesse-Darmstadt had produced more sons (Louis, Philip (III) and Frederick) than William of Hesse-Cassel with just one heir, Maurice.

Following an action for annulment by Hesse-Darmstadt, the matter was decided in favour of Hesse-Cassel by the Aulic Council (Reichshofrat), and Hesse-Marburg was divided in half.

[3][7] In 1605, the dispute over the Marburg inheritance flared up again after Landgrave Maurice of Hesse-Cassel, whose beliefs since his accession in 1592 increasingly moved towards the Calvinistic confession of his wife, Juliane of Nassau-Siegen, enacted several Calvinist-oriented laws in his domain and in the same year, converted to Calvinism himself.

After the Protestant Duke Christian of Brunswick-Wolfenbüttel had invaded Upper Hesse in 1621, Louis V openly allied himself with the emperor in the hope of receiving military support.

In reply, the commander of the Protestant army, Ernst von Mansfeld, counter-attacked the Darmstadt upper county on orders of Prince-Elector Frederick of the Palatinate.

As a result, he enforced the Aulic Council's judgement of 11 April 1623, that the entire heritage of Hesse-Marburg (including all tax revenue from it, retroactively) should go to the Darmstadt line.

Because of military defeats and abject government by Landgrave Maurice of Hesse-Cassel, the Lower Hess estates openly rebelled against him and forced his abdication in 1627.

After the total defeat of the House of Hesse-Cassel had been averted by the Hauptakkord, in 1627 William V began secretly waiver, to build a new mercenary army, under the cover of his apparent renunciation.

Instead, on 28 February 1632, Gustavus Adolphus granted Hesse-Cassel several other areas outside of Hesse that the Lower Hessian troops had conquered earlier on Swedish orders (including the Fulda Abbey, the Bishopric of Paderborn and Corvey Abbey) or intended to capture (the Bishopric of Münster, later substituted by the Swedish Chancellor Oxenstierna for parts of the Duchy of Westphalia and Vest Recklinghausen).

After the Battle of Lützen in November 1632, in which the Swedish-Protestant side suffered great losses and King Gustavus Adolphus was killed, the fortunes of war turned against the Protestants, including Hesse-Cassel.

In 1641 Hesse-Cassel lost the town of Dorsten in Vest Recklinghausen, which they had seized in 1633, to troops of the Imperial Army and Electorate of Cologne after a siege lasting several weeks.

But after the imperial forces partially withdrew to fight in other regions (especially Wolfenbüttel), Hesse-Kassel embarked on a campaign in the electoral lands on the left bank of the Rhine.

Inspired by military and diplomatic successes in the Rhineland and Westphalia, Landgravine Amalia Elisabeth of Hesse-Cassel felt strong enough to take up the fight for the Marburg inheritance in 1644.

She had the Hauptakkord treaty of 1627, in which Hesse-Cassel had forfeited Upper Hesse, invalidated by a subsequent legal ruling and, at the end of 1645, sent her battle-hardened troops led by Johann von Geyso towards Marburg.

Before the Westphalian Peace Treaty, Cassel fought one last time against the imperial side, winning the Battle of Wevelinghoven in the Rhineland, together with other Protestant troops.

Darmstadt had to give up a significant part of Upper Hesse to Cassel, not least Marburg and other occupied territories, including the Lower County of Katzenelnbogen and the Barony of Schmalkalden.