ORIGINAL: Canoerebel
100% prep helps, but it isn't essential in every case (though it was in this case, where the enemy is well prepared and has many forts). If you can catch the enemy by surprise so that a base isn't adequately garrisoned, or if you have so much time that you can blugeon it to submission, then it's okay to to go in with substandard prep.
The thing with prep is you pay now or you pay later. You can wait for 90-100% and be fine in exchange for the time cost, or you can go early and get disrupted/disabled on the landing, and pay later to recover. Atolls are a bit different in that you can go in with regiments, but to the west are islands that take multi-divisions. I've often disabled circa 150 squads of an ID landing with 60-70 prep. And even with massive supply and rest it can be two months to put the ID back to battery.
In my first PBEM, with the AAR, my opponent landed at Eniwetok with almost no prep, full IJA ID. It was wiped out to a man by a minimal defense. I think a para unit, a Seabees unit, and a base force, with Forts 3 or 4. He was quite upset.
Taking some tanks--any tanks--along on an atoll can sometimes save your bacon if the infantry is not prepped or is too small. You can withstand the auto-shock a lot better with some tanks in the first wave.






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