Dettingen; a formal alliance with Maria Theresa followed in the autumn; France responded with a secret alliance with Spain; and to prevent further British action on the Continent, she resolved to strike a blow at London in combination with a Jacobite insurrection.  It was to be a “bolt from the blue” before declaration and in mid-winter, when the best ships of the home fleet were laid up.  The operation was planned on dual lines, the army to start from Dunkirk, the covering fleet from Brest.

The surprise was admirably designed.  The port of Dunkirk had been destroyed under the Treaty of Utrecht in 1713, and though the French had been restoring it secretly for some time, it was still unfit to receive a fleet of transports.  In spite of the warnings of Sir John Norris, the senior admiral in the service, the assembling of troops in its neighbourhood from the French army in Flanders could only be taken for a movement into winter quarters, and that no suspicion might be aroused the necessary transports were secretly taken up in other ports under false charter-parties, and were only to assemble off Dunkirk at the last moment.  With equal skill the purpose of the naval mobilisation at Brest was concealed.  By false information cleverly imparted to our spies and by parade of victualling for a long voyage, the British Government was led to believe that the main fleet was intended to join the Spaniards in the Mediterranean, while a detachment, which was designed to escort the transports, was ostensibly equipped for a raid in the West Indies.

So far as concealment was concerned the arrangement was perfect.  Yet it contained within it the fatal ingredient.  The army was to strike in the Thames at Tilbury; but complete as was the secrecy, Marshal Saxe, who was to command, could not face the passage without escort.  There were too many privateers and armed merchantmen always in the river, besides cruisers moving to and fro on commerce-protection duty.  The division, therefore, which we supposed to be for the West Indies was to be detached from the Brest fleet after it entered the Channel and was to proceed to join the transports off Dunkirk, while the Marquis de Roquefeuil with the main fleet held what British ships might be ready in Portsmouth either by battle or blockade.

Nothing could look simpler or more certain of success.  The British Government seemed quite asleep.  The blow was timed for the first week in January, and it was mid-December before they even began to watch Brest with cruisers regularly.  On these cruisers’ reports measures were taken to prepare an equal squadron for sea by the new year.  By this time nearly twenty of the line were ready or nearly so at the Nore, Portsmouth, and Plymouth, and a press was ordered to man them.  Owing to various causes the French had now to postpone their venture.  Finally it was not till February 6th that Roquefeuil was seen to leave Brest with nineteen of the line.  The news reached London on the 12th, and next day Norris