Keep in mind that in World War II, the Japanese had lots of bases on little tiny islands. And it took hundreds of thousands of Marines to take them away. The US then took advantage of these islands to support attacks on other islands and, eventually, the Japanese mainland.
Taking them as part of the "island-hopping" strategy (which was a good way to advance to the Japanese home islands) was expensive, although not "hundreds of thousands". Iwo Jima, one of the bloodiest of those battles, was fought by 70,000 US troops, which is still a large number, of course. Neutralizing them was much simpler. The kingpin of the Japanese bases guarding the Central Pacific was an island called Truk. You've never heard of the great battle of Truk, because there wasn't one. We blockaded it, staged a massive bombing raid to destroy the aircraft and warships stationed there, and rendered it completely irrelevant. No attempt was made to take the heavily fortified island itself (which would have been a much tougher task than taking the islands we did take), because it wasn't necessary. Truk never fell; it was still under Japanese control when the Japanese surrendered. But without supplies and replacements for the destroyed planes and ships, it could no longer affect the course of the war.