Explain the reasons behind the quest for centralized imperial rule and the introduction of islam to northern india

When the empire of Harsha derelict the local rulers once again turned northern India into a contested regions they sought to expand their realms at the expense of their neighbors. When they were trying to expand the umayyad empire Islam reached through military power. Muslim merchants took them trust to coastal regions. Muslim merchants established small communities in the major cities of coastal India. Migrations and invasions also brought Islam.