It was principally to prevent this result that Richelieu after the battle of Nordlingen threw himself into the struggle, but his aim was also to carry the frontier of France up to the Rhine.

Here the territories of the Dukes of Lorraine, and Bouillon Prince of Sedan, not only cut France off from the Rhine and the Moselle, but opened a door by which she could at any time be invaded from Germany.

The Dukes of Lorraine had always borne themselves as independent princes, giving, indeed, a nominal allegiance to France, but as often allying themselves with German princes as with her.

The Duc de Bouillon, on the north of Lorraine, and the Duke of Savoy, farther to the south, also regarded themselves as independent.

The former, as Huguenots, had a strong leaning towards the Protestant Hollanders, and both were ready to furnish asylums to French nobles who had incurred the wrath of their kings or ministers.
The Duc de Bouillon, father of Turenne, had fought bravely on the side of Henry of Navarre through the wars of the League.