In World War II's Pacific Theater, the United States had a big problem: the operating area was humongous. In one sense, it's no surprise — the Pacific is the world's largest ocean and they needed to get across that ocean in order to defeat Japan. But Japan had also occupied a lot of bases in the Gilbert and Marshall Islands during the inter-war period (and illegally fortified them). Finally, the Allies needed a way to deal with the fierce Japanese force, but they needed to do so without endangering the "Germany first" grand strategy for defeating the Axis.
This problem proved extremely difficult. The Japanese, at Guadalcanal, in the Philippines, and elsewhere, had proven to be fierce fighters on the ground. It was painfully obvious that fighting island to island on a campaign across the Pacific would take a lot of time and cost many lives. But at the same time, the Japanese bases had to be neutralized.
In 1943, after Guadalcanal had been cleared, Admiral William F. Halsey and General Douglas MacArthur began planning the next phase of the offensive in the massive ocean, with the ultimate objective of taking out Rabaul, Japan's major base in the south Pacific.
The first plan they came up with would have required additional forces drawn from efforts in Europe. That, of course, didn't fly with politicians.
Douglas SBD Dauntless dive bombers fly over an atoll in the Pacific during the island-hopping campaign.
Instead, the answer to the Pacific question was to grab a few key bases and then use air power and submarines to cut off the other Japanese installations from resupply and reinforcement. The term for this was "island hopping" or "leapfrogging."
There were two primary benefits to this strategy: First, it could be accomplished with fewer troops. Second, it meant the cut-off enemy forces couldn't be pulled back to reinforce important objectives, like the Philippines.
Bases seized by the Allies were used to launch strikes that targeted enemy supply lines. One of the most famous actions was the Battle of the Bismarck Sea.
The targeted bases in the island-happen campaign were selected for two purposes: First, they were the jumping-off points for the next "hops" towards Japan. Second, they served as bases for forces that had the job of plastering the now-isolated garrisons left behind. This was what John Glenn did while serving in World War II.
While plans originally called for capturing Rabaul, the decision was made to bypass it after successfully seizing some other locations where Allied forces could build airfields.
John Glenn's World War II service included a combat tour striking bypassed Japanese garrisons in the F4U Corsair.
The island-hopping strategy worked. In less than four years, the United States had forced Japan's surrender. While much of history focuses on the hotly-debated use of nuclear weapons on Hiroshima and Nagasaki, the ability for America to deliver those weapons hinged on some very strategic leapfrogging.