Final answer:
The Battle of Guadalcanal was the prolonged conflict that indicated to Marines the Pacific War would be extensive. Despite the US victory at Midway, which marked a turning point, the grueling campaigns on islands like Guadalcanal and Iwo Jima showed the tough path ahead.
Step-by-step explanation:
The battle that made it clear to Marines that the war in the Pacific would be a protracted one was the Battle of Guadalcanal. This intense conflict took place over six months from August 1942 to February 1943, symbolizing the beginning of a long and grueling campaign ahead for American forces. Engaging the Japanese in dense jungle terrains with fierce Japanese resistance amplified the difficulty.
The Battle of Guadalcanal, part of the larger island-hopping strategy, marked a challenging phase wherein neither side could gain a definitive advantage for some time, leading to a stalemate. This battle reflected the Japanese military doctrine that relied heavily on 'heroic determination' to overcome adversaries, but ultimately, they were pushed back.
A reflection of the relentlessness was also seen in Iwo Jima, where despite overwhelming American numbers, the Japanese held on for over a month, causing significant US casualties. This phase of the war, known as the Allied offensives, lasted from mid-1943 until the end of the war, with pivotal battles such as the Battle of the Philippine Sea and Battle of Leyte Gulf. These combats gradually edged the Allies toward victory but showcased the rigorous path that lay ahead.
The turning point in the Pacific came earlier at the Battle of Midway in June 1942, which marked the end of Japanese naval dominance but not the end of the violent and protracted ground battles that Marines would face on the islands en route to Japan.