Why is iwo jima considered one of the bloodiest battles that occurred on the pacific front?

Because the combat, planned by strategists to last less than a week, lasted thirty-five days, becoming one of the fiercest and bloody battles of the Pacific War.  

Moreover, the aftermath of the war was disastrous. Of the 20,530 to 21,060 Japanese soldiers entrenched on the island, between 17,845 and 18,375 have died (most fighting, though some are by suicide). Only 216 were captured during the battle. By the time the battle had been declared finished by the Americans, it was estimated that there were about 300 Japanese still in Iwo Jima, hidden in tunnels and caves. In fact, there were at least 3,000 of them hidden.

Although victorious, the Americans paid a heavy price for the conquest of Iwo Jima. According to official data from the Navy Library, "the 36 days of battle (at Iwo Jima) resulted in 26,000 American casualties, including 6,800 dead and nearly 20,000 wounded." By comparison, the 82-day Battle of Okinawa from April to June 1945 (involving five US Army divisions and two Marine Corps divisions) resulted in 62,000 US casualties, including 12,000 men killed or missing.

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