Recent scholarly debate about the Glorious Revolution has put renewed focus on the fear of a new aggressive Catholic confessionalism that was widespread among English and European Protestants. One important example is the threat of an imminent French-led joint Catholic aggression against the Netherlands and other Protestant states. This fear was shared by William of Orange and contributed to his decision to risk invading England in the autumn of 1688. Thanks to new archival sources, it is clear that Emperor Leopold contributed substantially to increasing this fear. In July 1688, the imperial government informed William of Orange about unprecedented French offers to Leopold to win over the emperor for a new Catholic alliance. Almost certainly these offers were fictitious, but nevertheless they had an alarming effect on William: he was convinced that an autonomous, ‘uncontrolled’ development in England (regardless of whether it would lead to a ‘popish’ despotism or to a Protestant republic) would only benefit France and should be avoided in this decisive situation. Consequently, after July 1688 William and his diplomats repeatedly referred to the supposed ‘indiscretions’ from Vienna to demonstrate the necessity of intervening in England.