Ah yes, the favorite topic of Alternate Timelines on the forum. What would have happened to this Empire if it hadn't been consumed by nuclear fire in the 1970's? We all know about the unholy trinity by now. It inherited its naval power from the British Empire after its collapse, early in its history it had the world's best cavalry force (thanks to it's cavalry tactics and the vast pastures it controlled) and its mastery of siege warfare. It's often been said that it was the combination of the Viking's naval dominance with the Mongols land dominance.
Despite that there were a few problems towards the end. The Kalgoorlie uprising got within a few kilometers of its capital at Perth. They lost the Terra South East coast to Germanic invasions. Northern Terra was depopulated by invaders at the beginning of the late Terran period.
It's rise to power was a bit strange too. With enough universes, unlikely things start to occur in at least a few of them. People have been saying that its improbable rise to power may mean we live in that part of the Multiverse.
OOC: Terra comes from the Terra in "Terra Australis"