Hours after attacking Pearl Harbour in 1941, Japanese forces annexed which USA territory?

Answer 1

Guam
Japan did occupy the Philippines but not so soon. US forces retook Guam in 1944. The Northern Marianas were under Japanese administration already, and were the site from which Japan launched its attack on Guam. The US captured the Marianas a month before retaking Guam in 1944, and it remains (like Guam) and unincorporated US territory.