Geography.

Prior to it's invasion of Japan, Britannia was shown to include all of the western hemisphere (Both North and South America) as well as New Zealand and the Philippines. The Novels also show Indochina (Area 10) as belonging to the Empire, though other maps contradict this. Oddly enough, the British Isles are not part of the Britannian Empire but the EU, being lost to the mainland during the Napoleonic wars. Australia remains independent.

In the premier introduction, Britannia invades Japan, one of the few other independent nations, in order to secure Sakuradite mineral deposits.

Britannia successfully conquers the majority of the Middle Eastern Federation, renaming it Area 18. At the Start of the Second Season, Prince Schneizel leads Britannian Forces against the EU, which held all of Africa, Europe, and Central Asia, successfully conquering almost half their territory, though what half is unknown.

Currently, the Britannian Army, under Prince Schneizel is attempting to annex provinces of the now disintegrating Chinese Federation, adding even more territory.