Following the death of King Louis XIV, power in France fell to the Regent, Philippe II, Duke of Orléans, while King Philip V of Spain harbored intense ambitions to claim the French throne for himself. Seeking to overthrow Orléans, Prince Antonio Giudice de Cellamare, the Spanish ambassador to Paris, orchestrated a sweeping diplomatic conspiracy. Cellamare allied with discontented French aristocrats, including the Duchess of Maine, planning to kidnap the French Regent and declare Philip V administrator of France.
However, the conspirators committed a fatal security error. Instead of using standard diplomatic ciphers or complex codes, the Spanish embassy dispatched couriers carrying plain-text documents detailing the entire plot, including lists of conspirators and troop movements. French intelligence, which maintained an efficient mail-interception system known as the Cabinet Noir, intercepted the couriers near Poitiers.
Upon reading the documents written in plain Spanish, French authorities easily unraveled the coup. The Regent reacted swiftly by arresting Cellamare, expelling Spanish diplomats, and locking up the involved French nobles in the Bastille. The blunder directly triggered the War of the Quadruple Alliance, uniting major European powers against Spain and permanently securing the French Regency.