It was based on a 1906 story by William le Queux, The Invasion of 1910, about the cruelty of the "Nordener" army as it swept across the English countryside. The 1914 film depicted the executions of resisters at Beccles and the last stand of the British Army at Maldon, in Essex, where their central position was defended by the very trenches and barbed wire entanglements with which we would soon be so familiar.
It was submitted to the British Board of Film Censors as The Raid of 1915 and was rejected as offensive to a foreign power. When war was declared, it was hurriedly released under its new title.
Britain's paranoia dates back to the crushing defeat of the French in 1871.