The idea of a caliphate has a powerful history for Muslims, who enjoyed their greatest political and military successes under ancient caliphs. But Muslims are divided over whether the caliphate proclaimed by ISIS militants is worthy of that proud mantle.The idea of a caliphate has a powerful history for Muslims, who enjoyed their greatest political and military successes under ancient caliphs. But Muslims are divided over whether the caliphate proclaimed by ISIS militants is worthy of that proud mantle.