North Yemen (Yemen Arab Republic) was mostly Shia and South Yemen (People's Democratic Republic of Yemen) was mostly Sunni. What factors led to the merger of the two countries in 1990?
Well both countries agreed pretty early on their indepence that they should unite.Unlike other countries divided in two like North and South Korea, North- and South Vietnam and East and West Germany, the two countries had pretty good relations and they didn't want to be separate. If i remember correctly they agreed about the unification already in 1979 after some border skirmishes and then the berlin wall fell so they did it.