Months after declaring an Islamic caliphate, Islamic State, which has seized large swaths of Syria and Iraq, is seeking to address a need of any viable nation: women. In Internet posts and social media messaging, the extremist Sunni militants are recruiting women to marry their fighters and have children, part of a larger strategy of state-building. "They are treating the Islamic State as a country that needs women," said Rita Katz, director of the SITE Intelligence Group, which monitors online activity by militant organizations. "The message is: 'You are coming to marry someone immediately and have kids and cook.' They're...