Why did the Duke d’Enghien command the combined French armies near Freiburg in 1644?

In 1644, the Duke d’Enghien (Louis II de Bourbon, prince de Condé) commanded the combined French armies near Freiburg because, as a French prince, he held higher rank than the French marshal Henri de La Tour d’Auvergne, vicomte de Turenne.1

Turenne, who was commanding the French forces in Germany, had appealed for assistance after the Bavarians captured Freiburg im Breisgau. The Duke d’Enghien arrived with his own small army to join Turenne’s forces, and despite being ten years younger than Turenne, he assumed overall command due to his royal status. Although they were colleagues, this seniority dictated the leadership structure during the Battle of Freiburg, a series of fierce actions that ultimately forced the Bavarian-imperial armies to withdraw from the Rhine River valley.12