The country of Belgium derives its name from Gallia Belgica, the Roman province. This led to "België" in Dutch, "Belgique" in French, and "Belgium" in English.
Why was the Latin word Anglicized this way? I know of no other "-ium" countries, but plenty of "-a" and "-ia" countries. So why wasn't "Belgica" left as-is for the new country's English name? It's a perfectly comfortable pronunciation for English-speakers. Or why not "Belgia" or "Belgicia," to match the common "-ia" pattern?
Wiktionary gives Belgium as the proper declension of Belgae, the tribe for whom the province was named. This makes sense, but it would seem to be the only example in all the world. Were no other countries named for tribes known to the Romans? Or were they all Anglicized by a different pattern?
One can speculate that it was a juvenile suggestion by some starhopping truant in 1830, the word "Belgium" being terribly offensive in the rest of the galaxy.