We are coming up on the 68th anniversary of the 36 day battle of Iwo Jima on Feb. 19th. The shocking casualty statistics were overshadowed by the Uncommon Valor of the US Marines. Almost 7,000 Marines killed and about 26,000 wounded in 36 days. The amazing Joe Rosenthal photo of the Flag raising on Mt Suribachi was the most copied photo in history but the event was just the beginning of the battle. In a week several of the Flag raisers would be killed. The big question has always been, was it worth it. You have to consider that the original mission was to take the airfield and suppress Japanese fighter planes from harassing US bombers. After the shocking statistics became evident the government revised the original mission parameters and claimed that the intent was to use Iwo Jima as a landing site for crippled bombers. Either US intelligence was profoundly faulty about Japanese resistance or the Marine assault was a training experiment for the invasion of the mainland or the Navy Dept didn't give a damn how many Marine lives it would take so that crippled Bomber crews would have a convenient landing site. Either way the Marines got the shitty end of the stick.
You may well ask that same question about the entire Pacific campaign and many have. Most, of it all, those island fortresses could have been bypassed as well as assaulted. But, politics reared its head early on.
First of all, in the years before the war, the Marine Corps was in danger of being disbanded because it was seen as a superfluous force. Why did we need two ground components? The original need for Marines was past history. So, in order to save their force, the Marines came up with a new reason for being: They designated themselves as amphibious experts and sold that to Congress, in spite of the fact that the Army had been conducting amphibious operations since at least the War of 1812 and, even today, have mounted far more and far larger operations.
Additionally, the Navy insisted that they did not want to serve under the Army in the Pacific. While they accepted McArthur as the over-all commander, they successfully sold FDR on the idea of a two-pronged assault on Japan in order to use their forces in the manner which they saw fit. The result was that the Army had the primary responsibility for the liberation of the Philippines and the Navy would take off across the central Pacific, reducing or bypassing Japanese strongholds along the way. Looking back, it's possible to see that the Navy's campaign was perfectly useless. McArthur knew it at the time, but was over-ruled in Washington.
The Navy needed to find a target to prove their new doctrine and the first opportunity was Tarawa. Instead of adopting the lessons learned from generations of Army amphibious operations, they wrote their own and the results were little short of a total disaster. Yes, they took the island, but questions were raised as to whether or not it was worth it. Those questions would be asked after every major invasion in the central Pacific because the casualty figures were invariably compared to the succession of Army assaults on the way to Manila and the Navy's casualty figures far exceeded the Army's in every instance.
Why it continued for another 2 years is politics, pure and simple.