From StrategyPage in a discussion about the civil war in Yemen:
For thousands of years Arabia was largely poor and thinly populated. Most of the population and wealth (what we now call GDP) were in the south, where there was regular rainfall in what is now Yemen. During all that time Iran was the regional superpower.
Then came oil to complicate matters and Yemen became the poor cousin.
I was not aware, however, that the Persians were not always Shiites: it was imposed on them.
The Turks were relatively new in the region and had adopted Sunni Islam. Iranian rulers deliberately forced their subjects to convert and that process took over a century to complete. That produced an ethnic/religious divide in the region which is now triggering and sustaining several wars, including the one in Yemen and a larger potential one between Saudi Arabia and Iran.
so although the Persian/Arab conflict goes back to Darius' time, the Shiite conflict only dates back to 15th century... WIKIPEDIA has more details.