Which was the last island that the Allies captured before preparing to attack mainland Japan?

Iwo Jima was the last Island the allies had captured before preparing an attack on mainland Japan.

Before the complete mainland attack on Japan, the allies deemed it important to capture strategic islands around Japan in order to ensure proper lines of supply and communication.

Capturing Iwo Jima was seen as important step and this resulted in the Battle of Iwo Jima.

The Battle was one the bloodiest in the Pacific Theater and was only a narrow win for the Allies. However, after the battle ended, the strategic important of the Island was widely questioned.

Today, it is often described as a controversial war, where 26,000 + soldiers were killed for very little real gain.

Iwo Jima was the last captured island before the attack on mainland Japan.