Okinawa showed otherwise. That's why it was decided to drop the nukes, imagine the whole fight for Japan being like it was on Okinawa, it would have been an absolute slaughterhouse.
The really horrible losses of men during that war was because we had a tight timetable: we were in a hurry to get the war over with before the American people stopped supporting it.
Thanks to stupidity of our leadership, lessons learned were always after the fact and assaults carried out with little or no idea of what the enemy had waiting for us. Some of the objectives - Peleliu and Iwo - weren't even really necessary, yet thousands of good, motivated Marines were lost forever.
We were always told that succeed in our mission and if you can, save the lives of your troops. That is only possible if the folks in charge bother to find out what the enemy fortification systems, fire support and positions really are before launching the attack.