Why were the battles of iwo jima and okinawa so challenging? what did the fighting there suggest for an invasion of the japanese mainland?

One of the reasons the United States Marines have always been considered the most elite fighting force in the world is because not only is every individual a weapons expert and trained in hand-to-hand combat, but they are are also made extremely proficient in adapting and overcoming any situation in pretty much any terrain. Although they were trained in these tactics nothing could prepare them for the volcanic bedrock terrain of Iwo Jima. The physical foreign theatre wasn't their only challenge. They also had to basically bum-rush their way through certain death in both Iwo Jima and Okinawa meeting heavy resistance with machine gun fire, which made it even more difficult to reach an enemy that was dug in with caves, tunnels, and holes. We hear about these experiences now in history class and on television documentaries as if it's common knowledge, but the reality was that those marines had no idea where the rounds were coming from when they first landed, which shows us to never underestimate the enemy. During the battle of Okinawa, and also throughout the course of the war, allied ships off shore were bombarded with kamikaze attacks (Keegan 567). These battles told us not to invade the mainland because we would have catastrophic losses on both sides. I strongly believe that if we hadn't reverted to atomic power against such a nationalistic force we never would have saw victory. It's absolutely nuts that it took a second bomb for them to even consider surrender ...