Antebellum means ‘from the time before a war’. The Latin is plain: ante (before) + bellum (war). In English, though, it is not a free word for ‘before just any war’. Americans almost always use it about one war — the American Civil War (1861–1865). Antebellum houses, the antebellum South, antebellum days: these point at the decades before that war, especially in the southern states, with the big plantation houses and, though tidy tourist talk sometimes skips this, the system of slavery that sat under a lot of that wealth. It is not a general synonym for old-fashioned, and it is not ‘anti-war’. Outside US history it sounds marked, a little literary. If you mean ‘before the Second World War’, say that; antebellum will send people to America in the 1800s.