Peleliu and Okinawa from the Pacific theater of WW2 are pretty fascinating to me. Peleliu had a harrowing beach assault, worse than Omaha beach even from what I hear, but both battles marked a change in Japanese defense tactics, where before they'd been obsessed with repulsing the American Marines at the beach without much in the way of a backup plan, but then they switched to an in-depth strategy, turning tiny islands into impenetrable fortresses that led to what sound like the worst things anyone has ever gone through since the trenches of WW1. Totally waiting for those movies.
https://en.wikipedia.org/wiki/Battle_of_Peleliu
https://en.wikipedia.org/wiki/Battle_of_Okinawa