In April 1758, the British concluded the Anglo-Prussian Convention with Frederick in which they committed to pay him an annual subsidy of £670,000. Britain also dispatched 9,000 troops to reinforce Ferdinand's Hanoverian army, the first British troop commitment on the continent and a reversal in the policy of Pitt. Ferdinand had succeeded in driving the French from Hanover and Westphalia and re-captured the port of Emden in March 1758 before crossing the Rhine with his own forces, which caused alarm in France. Despite Ferdinand's victory over the French at the Battle of Krefeld and the brief occupation of Düsseldorf, he was compelled by the successful manoeuvering of larger French forces to withdraw across the Rhine.
If it is possible to answer this question, answer it for me (else, reply "unanswerable"): What caused alarm in France?
re-captured the port of Emden in March 1758 before crossing the Rhine with his own forces