Thwarted by La Manche and the superiority of the Royal Navy, the French armed forces invested much effort into developing ways of circumventing this problem to landing troops on British soil. The obvious solution was through the use of aeronef and aerostat, although the Great War of 1890-91 demonstrated that this was only viable if the French (and their allies) could achieve aerial superiority, which they proved unable to do.
French scientists looked at other options and inspired by the machinery being used in the creation of 'Le Tunnel Sous La Manche' soon developed contraptions capable of burrowing underground. At first these machines were quite small, capable of only transporting a section of men, but once the concept was proven as sound, larger machines were created capable of carry whole companies and squadrons.
Unfortunately for the French, an enemy spy, Captain Alfred Dreyfus, stole plans of these designs and soon all the major powers embarked on the development of their own terranef (as they have generally become known as).
Utilised widely in the Slavo-French invasion of Britain in 1899, most notably in the seizing of Dover Castle and the over-running British defences to the Channel Tunnel before it could be flooded, terranef are a formidable weapon and have most recently been used by Great Power forces in the battles on Mars at Auorae Sinus.