Complex sentence: After Fath Ali Khan's death the khanate's influence declined as a result of Agha Khan Qajar's conquests and devastation its brought, and alliance of Northern khanates disintegrated.
Split into simpler sentences: After Fath Ali Khan's death, the khanate's influence declined. As a result of Agha Khan Qajar's conquests and the devastation its brought, the Alliance of Northern khanates disintegrated.