Given the task definition and input, reply with output. This task is about reading the given passage and construct a question about the information present in the passage. Construct a question in such a way that (i) it is unambiguous, (ii) it is answerable from the passage, (iii) its answer is unique (iv) its answer is a continuous text span from the paragraph. Avoid creating questions that (i) can be answered correctly without actually understanding the paragraph and (ii) uses same words or phrases given in the passage.

Calculating that no further Russian advance was likely until 1758, Frederick moved the bulk of his eastern forces to Pomerania under the command of Marshal Lehwaldt where they were to repel the Swedish invasion. In short order, the Prussian army drove the Swedes back, occupied most of Swedish Pomerania, and blockaded its capital Stralsund. George II of Great Britain, on the advice of his British ministers, revoked the Convention of Klosterzeven, and Hanover reentered the war. Over the winter the new commander of the Hanoverian forces, Duke Ferdinand of Brunswick, regrouped his army and launched a series of offensives that drove the French back across the River Rhine. The British had suffered further defeats in North America, particularly at Fort William Henry. At home, however, stability had been established. Since 1756, successive governments led by Newcastle and Pitt had fallen. In August 1757, the two men agreed to a political partnership and formed a coalition government that gave new, firmer direction to the war effort. The new strategy emphasised both Newcastle's commitment to British involvement on the Continent, particularly in defence of Germany, and William Pitt's determination to use naval power to seize French colonies around the globe. This "dual strategy" would dominate British policy for the next five years.
Why did Ferdinand feel comfortable withdrawing troops from the eastern front?