Complex sentence: The book uses geography to show how Europeans developed such superior military technology, and how Europeans and Asians developed some immunity to diseases which spread among them, while epidemics of them devastated the indigenous populations in the Americas after European contact.
Split into simpler sentences: The book uses geography to show how Europeans developed such superior military technology, and how Europeans and Asians developed some immunity to diseases which spread among them. However, such diseases led to epidemics which devastated the indigenous populations in the Americas after European contact.