What were some of the geographical and political reasons which explain how Alexander was able to assemble such a vast empire in such a short time and why his successors were unable to hold it together?

Alexander the Great

Already selected 5 sources as required by my professor and completed an annotated bibliography, which have attached -can use only those sources. The essay needs to be third person, 750-1250 words.

The prompt is:    What were some of the geographical and political reasons which explain how Alexander was able to assemble such a vast empire in such a short time and why his successors were unable to hold it together? Be sure to provide specific points to support your argument.