He sent his agent Vidomacq to the Landgrave of Hesse with instructions to do his utmost to confirm the princes of the Union in organized resistance to the schemes of Spain, and to prevent any interference with the Condominium.
He wrote letters to the Archdukes and to the Elector of Cologne, sternly notifying them that he would permit no assault upon the princes, and meant to protect them in their rights.

He sent one of his most experienced diplomatists, de Boississe, formerly ambassador in England, to reside for a year or more in the duchies as special representative of France, and directed him on his way thither to consult especially with Barneveld and the States-General as to the proper means of carrying out their joint policy either by diplomacy or, if need should be, by their united arms.
Troops began at once to move towards the frontier to counteract the plans of the Emperor's council and the secret levies made by Duchess Sibylla's husband, the Margrave of Burgau.

The King himself was perpetually at Monceaux watching the movements of his cavalry towards the Luxemburg frontier, and determined to protect the princes in their possession until some definite decision as to the sovereignty of the duchies should be made.
Meantime great pressure was put upon him by the opposite party.

The Pope did his best through the Nuncius at Paris directly, and through agents at Prague, Brussels, and Madrid indirectly, to awaken the King to a sense of the enormity of his conduct.
Being a Catholic prince, it was urged, he had no right to assist heretics.

It was an action entirely contrary to his duty as a Christian and of his reputation as Eldest Son of the Church.