In early June 1757, the French army began to advance towards Hanover once it became clear that there was to be no negotiated agreement. The first
skirmish between the two forces had taken place on 3 May. Part of the French army was delayed by the
siege of Geldern which took three months to capture from its Prussian garrison of 800. The bulk of the French army advanced across the Rhine, advancing slowly because of the difficulties of
logistics for moving an army estimated at 100,000. In the face of this advance, the smaller German Army of Observation retreated back across the
River Weser into the territory of the Electorate of Hanover itself, while Cumberland tried to ready his troops. On July 2, the Prussian port of
Emden fell to the French before a
Royal Navy squadron sent to relieve it could reach there. This cut Hanover off from the
Dutch Republic meaning that supplies from Britain could now only be shipped direct by sea. The French followed this up by seizing
Cassel, securing their right flank.
Battle of Hastenbeck By late July, Cumberland believed his army was ready for battle and adopted a defensive position around the village of Hastenbeck. The French won a narrow victory over him there, but as Cumberland retreated his force began to disintegrate as morale collapsed. Despite his victory, d'Estrées was shortly afterwards replaced as commander of the French army by the
Duc de Richelieu, who had recently distinguished himself leading the French forces that had
captured Minorca. Richelieu's orders followed the original strategy of taking total control of Hanover, and then turning west to offer assistance to the Austrians attacking Prussia.
Occupation of Hanover Cumberland's forces continued to withdraw northwards. The French pursuit was slowed by further problems with supplies, but they continued to steadily pursue the retreating Army of Observation. In an effort to cause a diversion and provide some relief to Cumberland, the British planned an expedition to raid the French coastal town of
Rochefort – hoping that the sudden threat would compel the French to withdraw troops from Germany to protect the French coast against further attacks. Under Richelieu the French continued their drive, taking
Minden and then capturing the city of
Hanover on 11 August. , the capital of the Electorate, was occupied by French forces in August 1757. Richelieu despatched a force to occupy
Brunswick. Frederick the Great then decided to withdraw the Prussian contingent of Cumberland's army so they could rejoin his own forces further weakening the Army of Observation. The Hanoverian government retreated with Cumberland via
Verden to
Stade,
Bremen-Verden's capital and a port town connected to the
North Sea by the
River Elbe. Although it was well-fortified and could be supplied from sea, Cumberland believed their situation to be precarious. The proposal that a large number of British reinforcements be diverted to Stade was rejected, and the
British expedition was sent to its original destination of
Rochefort, although it was launched too late to do anything to provide a diversion in support of Cumberland. Orders were sent to Captain
Hyde Parker to use his Royal Navy squadron to keep open the supply route down the Elbe to Cumberland until the onset of
ice prevented him. The deployment of this squadron left Richelieu to believe that Cumberland's position in Stade was secure, as he could not be defeated through lack of food and supplies. Morale in the French army had now collapsed, and many troops had been immobilised by illness. Richelieu was now open to a negotiated settlement – a prospect he had rejected on 21 August when Cumberland had proposed an
armistice.
Convention of Klosterzeven Frederick V King of Denmark was obligated by treaty to send troops to defend the Duchies of
Bremen and
Verden, both ruled in
personal union with Britain and Hanover, if they were threatened by a foreign power. As he was eager to preserve his country's neutrality, he attempted to broker an agreement between the two commanders. Richelieu, not believing his army was in any condition to attack Klosterzeven, was receptive to the proposal as was Cumberland who was not optimistic about his own prospects. On 10 September at
Klosterzeven the British and French signed the
Convention of Klosterzeven which secured the immediate end of hostilities. The terms called for several conditions. The national contingents from Brunswick and Hesse would return to their homelands. Half the Hanoverian force would be interned at Stade, while the remainder were to withdraw across the
River Elbe. Most of Hanover would be under French occupation, except for a demilitarised zone. The French would evacuate the Duchy of Bremen, provided the British withdrew their ships from the River Weser. However, a number of issues were left vague and poorly defined leading to later dispute and controversy. Following the convention the Army of Observation began to disperse, though they were not required to surrender their weapons. However some of the Hessians were forcibly disarmed by French troops, in apparent violation of the terms. The Convention was immediately attacked in Britain, and despite having given Cumberland authorisation to negotiate terms George II angrily rebuked his son on his return to London in October, compelling Cumberland to resign all his military offices. Richelieu also faced strong criticism from
Paris where the terms were perceived as far too lenient. He had decided it was too late in the year to now commit his forces to a full attack on Prussia which would have to wait until the following year. Instead he moved his forces to take up winter quarters around
Halberstadt. This was in spite of the fact he had instruction to besiege the Prussian fortress of
Magdeburg. The British government, seeing the terms agreed at Klosterzeven as contravening their own agreement with Prussia announced that they were not bound by it. They also began to lobby George II and his Hanoverian ministers to abandon the convention and re-enter the war. On 8 October George II revoked the agreement because of a technicality concerning the French interference with Hessian forces returning home. As the French were themselves dissatisfied by the agreement, they were also prepared to accept a return to hostilities. Directed by the British, the Hanoverians began to reform the Army of Observation and selected
Ferdinand of Brunswick, a Prussian commander and brother-in-law of
Frederick the Great, as its new commander. The British now agreed to take over the payment of Hanoverian troops as well as other contingents. This marked a sudden reversal in the policies of
William Pitt, as he had previously been opposed to further British financial commitments to a continental war. However, he still insisted that no British troops would be sent to join Brunswick's army. Ultimately this never came to pass, as British troops reinforced the Allied army at further battles, notably Wilhelmstahl. ==Allied counter-attack==