Victory at Waterloo

The victory at Waterloo in 1815 left Britain the dominant power in Europe with the Royal Navy the strongest fleet in the world - despite suffering a series of significant but small-scale reverses during the War of 1812 with the fledgling United States Navy. For 40 years threats of invasion were forgotten but then, in the late 1850s, emerged in a sudden and most dramatic manner. France - revived as an empire with immense territorial ambitions under Napoleon III - was once again the enemy and in the late 1850s Britain led by its Prime Minister Lord Palmerston undertook to spend vast sums on defence.

In 1859 a Royal Commission recommended the protection of Britain's main dockyards on both seaside and landward approaches with massive new forts being constructed at Portsmouth, Saltash, Plymouth, Milford Haven, Sheerness and Chatham. The total cost of these works - mostly completed during the 1860s - was a staggering $11.6 million, equal to around £520 million in modern money.

The speedy defeat of France by Prussia in 1870 and the ridiculous light it shed on the military worth of Britain's new and expensive generation of fortifications did not end the British fear of invasion. On the contrary, it merely identified a new enemy. Initially the British had been gratified by the discomfiture of their traditional enemy but by the end of 1870 Prussian brutality, its cold-blooded military efficiency and its territorial ambitions had made it the next potential invader.

(http://www.bbc.co.uk/history/british/empire_seapower/french_threat_01.shtml)