The beginning of the end of Alexander's conquests was his inability to fully subdue:

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Alexander faced significant resistance from the Bactrian tribes and was unable to fully conquer the region. This led to a decline in his army's morale and a reluctance to continue further eastward. Eventually, Alexander returned to Babylon where he fell ill and died, putting a halt to his conquests. While other regions may have also posed challenges, Bactria is generally considered to be the tipping point.

The Beginning of the End of Alexander's Conquests Was His