Final answer:
Alexander the Great's armies fought their way down the Indus river valley and crossed the southern part of the former Persian empire on their way back to Mesopotamia, but faced challenges when they reached the Indian Ocean.
Step-by-step explanation:
The Alexander in the given passage is most likely referring to Alexander the Great, the ancient Greek king. According to the passage, Alexander's armies had to fight their way down the Indus river valley and then cross the southern part of the former Persian empire on their way back to Mesopotamia. However, when they reached the Indian Ocean, Alexander made a major tactical error by splitting his forces into a fleet and a land force. The land force had to cross the brutally difficult Makran desert, which cost them more lives than the entire Indian campaign.