Descartes' correspondence during the two years prior to publishing the Principles is almost silent about the wider political and military turmoil within which he conducted his own intellectual war. Charles I had been king of England since 1625. He was formally challenged by Parliament in June 1642, in a document called the ‘Nineteen Propositions’, because of the arrogance and arbitrariness with which he allegedly exercised his royal powers. His formal reply, 18 June 1642, did little to satisfy his critics. The civil war that resulted brought Cromwell to power and, in January 1649, led to the public execution of the king by order of Parliament. During these tumultuous years, a number of royalist intellectuals emigrated to France and subsequently became correspondents with Descartes and, in some cases, supporters of his philosophy. They included Kenelm Digby and the two Cavendish brothers, William (marquis of Newcastle) and Charles.

The situation in the United Provinces was politically more stable than in England but, militarily, was just as unpredictable. The Stadtholder was constantly under pressure, both at home and on the battlefield, because he failed to synchronize military successes with political support, especially in the largest and most economically powerful province, Holland, as he carried on an increasingly stalled campaign against the Spanish Netherlands in the south.