Why take Betio?
Time.
By mid-1943 the Japanese had lost the initiative in the Pacific. The US controlled the Central Solomons and the Aleutians. Ahead lay northern Solomons, New Guinea and Bougainville.
We must remember that this was a global war - not just a Pacific War. Despite the enormous industrial production capacity of the US, it wasn't limitless. Africa, USSR and Britain had to be supported as well. There are many stories of Marines in the Pacific having to make do with pre-war equipment as production raced to provide the new arms in greater quantities.
The mood - in light of recent American victories - was one of attack! We believed we had wrested the initiative away from the Japanese and that the war was finally swinging our way. Admiral King was the primary proponent of an offensive campaign through the Central Pacific. Decisions made by the Combined Chiefs of Staff of US and UK during many meetings throughout 1943 lent greater support to more offensive campaigns in the Pacific.
The biggest decision ofcourse, was the postponement of the invasion of Festung Europa untill 1944. This meant that the massive amphibious resources needed for Operation Overlord were now temporarily available for use in the Pacific. Whatever offensive was about to be played out in the Pacific had to be sharp, quick and victorious - PACFLT commanders knew they were living on borrowed material on borrowed time. Any delays or a disastrous defeat would permanetly wreck a Pacific offensive campaign for a long period of time.
Now the question arises - Why Gilberts instead of Marshalls? The Marshalls were after all, greater in strategic value.
Believe it or not, one of the primary reasons was lack of good intelligence. After all taking an island is more than rowing a few dingies over and chucking bombs at the enemy and storming the beaches! Unfortunately, America at that point in time still had no effective way of garnering aerial intelligence - only Liberators could reach Marshalls but at that range they couldn't loiter. No fighters could protect them and carrier based aitcraft lacked this capability. Plus the Japanese had good airfields network. Were an assault to be mounted, the Japanese could easily reinforce from Truk.
No, the choice had to be the Gilberts.
And it had to be Betio because there was no time to land on the smaller islands, secure them, build airodromes and then pound/starve Betio into surrender/annihilation.
The biggest factors that resulted in massive casualties on Betio:
1. Lack of good intel on the tides. This completely and utterly wrecked the landing time table.
2. The initial assault bombardment was FUBAR. There was abysmal co-ordination between the Navy and Air Force.
3. Horrible communications. Radios consistently failed and for Day 1 and much of Day 2, there was little to no communication between forward commanders and rear support troops and reinforcements (witness how Landing Team 2/8th were slaughtered as they tried to make landings on Day 2 - sent to the wrong spot on the right beach thanks to miscommunication).
I must also mention that of all the island assaults, Tarawa was perhaps the Japanese's best and easiest chance of success. Nearly all accounts say so - to the point of saying that the American toehold at dusk on Day 1 was so tenuous that had the Japanese pulled even a Banzai, the Marines would have had no choice but to withdraw (or as Col. Shoup noted, "die where they stood rather than wade back under that murderous fire").
