Federal Republic of Nigeria
Who & why: Nigeria becomes a republic on 1 October 1963, replacing Queen Elizabeth II as head of state with a Nigerian president while remaining in the Commonwealth. The First Republic collapses in a January 1966 military coup, followed by a countercoup that July; worsening regional and ethnic tension leads the Eastern Region to declare itself the independent Republic of Biafra on 30 May 1967, triggering a civil war that runs until Biafra's surrender in January 1970. Nigeria returns to civilian rule in 1979, endures further military governments through the 1980s and 1990s, and has held continuous civilian democratic rule since 1999.
What it represents: the flag itself never changes through any of this — the same green-white-green tricolor adopted at independence in 1960 has flown over every government since, civilian or military, republic or the four years Biafra tried to leave it.