During this period the Twelve Years' Truce was negotiated, with French mediation, and Peckius tried to influence this process.

In 1620 he took part in the peace negotiations at Würzburg between the Bohemian insurgents and Ferdinand II, Holy Roman Emperor during the first stage of the Thirty Years' War.

In 1621 he was sent to The Hague by the government in Brussels to negotiate a renewal of the Twelve Years' Truce with the States-General of the Netherlands.

On travelling through Holland he was mobbed in Delft by hostile crowds, a breach of diplomatic immunity that his hosts claimed to have been powerless to prevent.

[1] Upon reaching The Hague he delivered a rather tactless address to the States-General on March 23, 1621, in which he proposed that the Dutch Republic would be left to run its own affairs in return for a nominal recognition of the sovereignty of the king of Spain.