In 1804 Napoleon Bonaparte was amassing an army of some 130,000 men, and a flotilla of some 2,000 boats, with the sole intention of invading and subjugating Britain. During this year, William Pitt the younger began his second term of office as Prime Minister, and began the task of finding a solution to the French threat. From the many suggestions put forward, two were ultimately adopted. One idea, proposed by Lt Col John Brown, was for the construction of a military canal across the low-lying marshland between the natural cliffs of Folkestone in the east and Hastings in the west, an area known locally as Romney Marsh.

Brown argued that a 62ft (18.6m) wide by 9ft (2.7m) deep canal should be dug from Shorncliffe to the River Rother, a distance of some 19 miles. On the northern side a rampart should be built using the spoil excavated from the canal, and beyond this there should be a military supply road. The canal should be constructed as a series of angular sections, with enfilading cannon fire covering each stretch. His plan was approved, and work commenced at the eastern end of the Royal Military Canal on 30th October 1804. Due to the harsh winter conditions and the ever-present problem of flooding, it soon became apparent that the work would take far longer than originally envisaged. Meanwhile, agreement had been given to extending the project with an additional section of Royal Military Canal to be built from the River Rother to the Pett Levels, east of Hastings. This resulted in the total length of the Royal Military Canal being increased from 19 miles to 28 miles, 22.5 miles of which had to be excavated by hand.

Work continued despite the difficult conditions, civilian labour being employed for the excavation of the Royal Military Canal, and military personnel detailed to concentrate on the rampart. In 1805 work also commenced on the second proposal adopted, which was to construct a chain of 'Martello' Towers along the south coast. Both projects together effectively turned the entire south-east coast into a building site. As work progressed the Royal Wagon Train (predecessor to the Royal Army Service Corps) pulled barges along the newly finished sections of Royal Military Canal to provide gravel for the construction of the military road. When Iden Lock was completed in 1805 it joined the Royal Military Canal to the river navigation, and effectively turned Romney Marsh into an island.

By 1809 the Royal Military Canal had been completed, although the cannons were not available until the summer of 1812. The final cost of the project had risen to a staggering £234,310. As so often the case when these large military projects were undertaken, the defensive structures became obsolete prior to their completion. In this instance, the likelihood of any invasion force finding a narrow stretch of canal much of an obstacle after having successfully negotiated the English Channel was almost ludicrous. Both these observations were not lost on the critics of the day.

Surviving today as a peaceful haven for ramblers, fisherman and wildlife alike, the Royal Military Canal is maintained along its length by the National Rivers Authority. It provides a valuable resource for the local farming community, preventing flooding in the winter and additional water supplies during dry summers. The present tranquillity of the Royal Military Canal perhaps provides us with a fitting epitaph to one of the most eccentric, and possibly ill-conceived, chapters in British military history.