Complex sentence: Twice during the Middle Ages, the Avars tried to conquer the Chechens, meeting fierce resistance, and more recently, in the 19th century, the ethnic Avar Imam Shamil forcefully added Chechnya to his Caucasian Imamate.
Split into simpler sentences: Twice during the Middle Ages, the Avars tried to conquer the Chechens, meeting fierce resistance. More recently, in the 19th Century, the ethnic Avar Imam Shamil forcefully added Chechnya to his Caucasian Imamate.