The Battle of Gaugamela (also known as the Battle of Arbela) was a decisive battle fought between Alexander the Great and Darius III of Persia on October 1, 331 BC, near the town of Gaugamela (or Arbela) in modern-day Iraq. The battle was a major turning point in Alexander's campaign against the Persian Empire and marked the end of Persian rule in the region. It is considered one of Alexander's greatest victories and is often referred to as his "Crowning Achievement".

Alexander had already defeated Darius at Issus in 333 BC, and now sought to end Persian rule once and for all. He gathered an army of more than 40,000 men and marched eastward towards Babylon. Darius had gathered an army of around 100,000 men, but was unable to match Alexander's tactics or maneuverability. When the two armies met at Gaugamela, Alexander used a combination of cavalry charges and infantry phalanxes to break through Darius' lines. Despite being heavily outnumbered, he was able to outmaneuver his opponent and eventually routed the Persian forces. The victory at Gaugamela marked the end of Persian rule in the region and enabled Alexander to pursue his goal of conquering all of Asia Minor.