Republic of Nicaragua (modern)
Who & why: the current flag and arms design is fixed by law in 1908 (proportions later refined in 1971) and survives every subsequent upheaval: the Somoza family dictatorship (1936–1979, propped up through most of that span by U.S. support), the 1979 Sandinista Revolution that overthrows it, and the 1980s Contra War between the Sandinista government and U.S.-backed rebels. Both the Sandinista government and its opponents fly the same national banner throughout — the flag itself is never a partisan symbol in these conflicts.
What it represents: the blue-white-blue bands descend directly from the Federal Republic of Central America's flag; the arms — five volcanoes (for the five federation states) beneath a rainbow and a liberty cap, with added rays of the rising sun — assert the same claim of federation heritage that Nicaragua's neighbors also carry.