I did some experimenting and careful reading of the manual, with some surprising results:
Situation: US wants to amphibious invade a pacific island that contains 2 (each of) Japanese aircraft, artillery, and infantry units. The US has a substantial (but not unlimited number) of aircraft, fleets, and land units and enough transports to support an amphibious invasion. The problem, unless I am missing something, is that if one conducts one giant amphibious invasion attack using all units simultaneously, the Op-Fire is resolved one invading land unit at a time before any fleets bombard or the aircraft attack. And a single defending unit can conduct multiple Op-Fire attacks. I have seen huge invading armies torn to shreds by just a few defending units capable of Op-Firing this way.
If the US uses its aircraft to conduct a preliminary airfield attack, it can likely eliminate the enemy air units, but leaves the artillery intact. And the fleets cannot bombard as part of this attack. And the enemy artillery are left intact to conduct Op-Fire at any followup AI.
If the US uses its aircraft to conduct a preliminary ground attack, they may be able to eliminate the enemy artillery units (which are targeted first), but only 1 fleet bombards as part of this attack. The rule on p.75 reads "The number of fleets that can bombard is equal to one plus the number of units amphibiously invading the region being bombarded." And of course the enemy aircraft are left intact to conduct Op-Fire at any followup AI.
An approach that seemed to work best was to conduct a sequence of air attacks against ground units, using 1 aircraft at a time. Each aircraft would automatically target an enemy artillery unit, and 1 fleet would also bombard an artillery unit on each such attack. After the artillery units are all eliminated, the remaining aircraft would collectively make an airfield attack, hopefully eliminating all enemy aircraft. If successful, I could then conduct an AI against no Op-Fire.
I believe the advantage of this latter approach is that it maximizes the number of fleets that can conduct a bombardment against enemy artillery before beginning an AI against possible Op-Fire. Maybe that is what you were saying, but it took me some time to figure that out, based on a close reading and understanding of the combat rules. And as you say, it is a good reason to upgrade the land attack capability of tactical aircraft. Thanks.