It refers to the Anti-Christ’s southern European campaign. His influence will begin spreading to where it will threaten Spain on the western side and threaten Romania on the eastern side. At this point in the campaign the Anti-Christ will have already taken over much of France and will prepare to take over Spain.
As for the «victory of the Insubrian campaign» you will find some parallels in the Gallic peoples and the Roman history when the Roman Empire was trying to expand northward into central Europe. The Romans were only able to go so far before they ran into problems. But the Anti-Christ will learn from history to overcome these problems. «The mountains of Hesperia» refer to the mountains of eastern Europe, particularly the Alps and the Caucasians. That area is mountainous and difficult to maneuver in.