This study represents the first published translation and analysis of an intriguing scheme of invasion of the British Isles that formed the foundation of all later invasion plans drawn up in the ivory towers of French diplomacy. References to invasion plans-made by Spain in the Spanish Armada (1585-98), or by the French Directory (1795-99) against Ireland and England in the later 1790s, or those of Napoleon Bonaparte (1799-1815), or ‘Operation Sea Lion, ‘ the German plan of invasion during the Nazi era-have been based on published information of earlier plans, at the heart of which was De Broglie’s grand project.