basically, the story is an alternate history of the H.R.E. Point of Divergence is in 1790. A 34-year-old Thurungian man named Vandox becomes Holy Roman Emperor. He embarks on several military campaigns during his 50-year reign. He essentially conquesrs the Italian peninsula, and makes France a protectorate after an 11-month war in 1796. In 1798, the Imperial Army invades BRitain.
thats where the story picks up, at the beginning of the invasion of England.