The U.S.-Japanese combined air-defense network was designed for a threat from China, to the west.

They get attacked from the other direction.

They did. Korea had been fully incorporated into Japan as a colony back in 1910. They bit off a chunk of China in 1931, and went back for the rest of it in 1937. At the time they attacked Pearl Harbor, they were still expecting to wrap that up Real Soon Now.

Also note they had invaded French Indochina, Hong Kong, Indonesia, Singabore, and other chunks of Dutch, French, and British territory. There was no effective resistance to any of that.

By the time they hit Pearl and the Philippines they’d wrapped up most of the Far East, and in 1942 they started moving into the ‘subcontinent’, starting with Burma. According to Churchill’s history of WWII, they frequently discussed the problem of defending India, Australia, and New Zealand, which was a logistical nightmare.

As often seems to be the case with conquests, the Japanese tried to bite off more than they could assimilate before the next wave of invasions. We know now that their conquest of China wasn’t nearly as complete as they thought, but their attack on the USA was their equivalent of Germany attacking Russia; a vastly larger opponents whose production heartland was beyond Japanese reach.