This is the Map of Bengal in 326 BC. Gangaridai (Greek: Γανγαρίδαι; Latin: Gangaridae) is a term used by the ancient Greco-Roman writers to describe a people or a geographical entity of the Ganges/Bengal Delta. Their writings describe  Alexander the Great withdrew from the Indian subcontinent because of the strong Military force of the Gangaridai  present Day Bengal  
Gnages Delta from space - On November 14, 2013 by NASA’s Terra satelliteAlexander crossed the Indus in 326 BC and fought and won an epic battle against King Porus, who ruled a region lying between the Hydaspes River Punjab, Therefore, Alexander turned south, advancing through southern Punjab and Sindh, along the way conquering more tribes along the lower Indus River.  After conquering industry Alexander persuade his soldiers to march farther to Ganges delta to conquer Gangaridai the most prosperous region in South Asia. 
According to Megasthenes, c. 350 – c. 290 BC)  and many Greek scholars Alexander army denied to attack Gangaridai for its geographic defensive position and its Mighty military power. 

City mark of  Upper ganges to Mouth of Ganges a  by Claudius Ptolemy  c. AD 100 – c. 170)
Gangaridai, a nation which possesses a vast force of the largest-sized elephants. Owing to this, their country has never been conquered by any foreign king: for all other nations dread the overwhelming number and strength of these animals. Thus Alexander the Macedonian, after conquering all of Asia, did not make war upon the Gangaridai, as he did on all others; for when he had arrived with all his troops at the river Ganges, he abandoned as hopeless an invasion of the Gangaridai when he learned that they possessed four thousand elephants well trained and equipped for war.---— Megasthenes, Indika
Own work based on: Ptolemy Asia detail.jpg  and Asia vectorial map.jpg
Aziz Tarak.