TL;DR: Regime in cambodia genocides about 1/4 of its population between 1975-79, is removed from power by Vietnam. UN helps recovery from destruction of society by previous govt & transition to new govt which after some initial chaos stabilizes. It's still not a proper democracy, but at least it isn't genocidal.

Considering the extremely difficult circumstances (a genocide so bad that it destabilized society & vietnam being an authoritarian regime), I am actually impressed by what the UN managed to pull off.