Complex sentence: By 1 September, the 1st Provisional Marine Brigade was down to 4,290 men, having suffered 500 casualties in its month of Korean service, and was preparing to move back to Pusan to evacuate to Japan.
Split into simpler sentences: By 1 September, the 1st Provisional Marine Brigade was down to 4,290 men, having suffered 500 casualties in its month of Korean service. At that point, it was preparing to move back to Pusan to evacuate to Japan.