In 1859 Lord Palmerston instigated the Royal Commission on the Defence of the United Kingdom because of serious concerns that France might attempt to invade the UK. Great improvements in gunnery (with RML and RBL designs) and the widespread introduction of steam propulsion in ships (the French La Gloire and British response HMS Warrior are examples) convinced him that Britain's coastal defences were inadequate to prevent invasion by Napoleon III if the Royal Navy were lured elsewhere.
The Royal Commission resulted in a huge wave of building around the navy's dockyards and harbours at Portsmouth, Chatham, Plymouth, Portland Harbour, Milford Haven and elsewhere.
By the time the fortifications were completed in the 1880s it was clear that the French had not seriously planned to invade, and the Franco-Prussian War of 1870-71 removed the threat, Palmerston was widely criticised, and these forts are often called Palmerston's Follies. As a key purpose of fortification is deterrent, this judgement was harsh, but with the rapid development of warship design at the time, the time taken to plan and implement the works would always have caused problems.
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