Many people, including the Mongols, but also including lots of people in Central and East Asia, embraced Islam without any military campaigns. And in fact, the Quran says that religion must not be an act of compulsion, but this much is true. The early Islamic empire was really good at winning wars. And situated, as they were, between two very wealthy empires, the Byzantines and the Sassanians, there was plenty to fight for. The first of all were the Sassanians, the last non-Muslim successor to the Persian empire. They were relatively easy pickings because they'd been fighting the Byzantines for like 300 years and they were super tired. Also they had recently been struck by plague.