If anyone is curious, there is a fantastic book about the Battle of Tarawa called "Utmost Savagery." Lots of interesting stuff in there.

American landing craft would get stuck on the reef, so Marines had to wade several hundred yards under high-caliber (12.7mm) machine gun fire. They got chewed the fuck up. Also, because this was one of the first amphibious landings on a heavily defended island, there were numerous other fuck ups. For example, there was lull of fifteen or twenty minutes between the end of the naval bombardment, and the beginning of the assault. The importance of those minutes for the defenders to reorganize, rush ammunition to where it's needed, etc, cannot be overstated.

The last point I'll make is this. You know that ratio they came up with for amphibious assaults? Something like 4 friendlies to each enemy soldier. Well, they hadn't even figured that out at this point, so the Marines on Tarawa were actually outnumbered for a while, until more of the landing craft made it.