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Japan needed Guadalcanal, as their other bases nearby would have been (and were) exposed to Allied strikes.
*if* (and this is a huge IF) the IJA and IJN would have supported each other, then the Allied hold on would have been tenuous at best. Add to that, a greatly improved operational strategy by IJN surface elements was needed.
I lost a paperback book written by a Japanese General officer who was against the occupation of the various islands scattered in outward perimeter around Japan but had wanted a closer in to Japan perimeter more heavily fortified. It was a good read with many quotes and foot notes. Unfortunately I was only half way through and can't remember title or author.
I had also read many in the IJA favored that strategy as well, but it also was fatally flawed. Let's say Japan stops at Truk in the Central Pacific, Rabaul in the South East. That puts Truk on the front lines. The US could have B-24s in the Marshall Islands bombing Truk by late 1942, early 1943. The US could have taken the Marianas in late 1943 and never would have had to base B-29s in China at all.
Like in the movie "War Games", the only winning strategy for Japan was to not play at all.
I believe that the Japanese 'disdain' for Allied fighting power and skill contributed greatly, at least in the beginning, to the piecemeal nature of their counteroffensive. It had taken much less effort than originally thought for them to crush the western powers in the south-pacific, south-asia area. The 'victory disease' (which Japanese officers described after the war) had overtaken the Japanese militaries to such an extent that an attitude of 'it won't take much to roust them from Guadalcanal' had permeated the upper ranks as well as the lower ranks. (re: Ichiki decides to attack without waiting for his whole regiment to be landed.) Underestimating one's enemies is dangerous in warfare and the Japanese leadership should have known better. It wasn't until the beginning of October '42 that the Japanese leadership finally realized the allies were going to fight seriously for the island and stronger and more serious plans for recapturing it were finally started. By that time we were too well dug in and had better logistics in place for them to throw us out without a massive effort.
I find it ironic that many of the bombs rained down on the Japanese occupiers in 1944 and 45 came from US aircraft flying from the very airfields which had been home to the initial Japanese attackers
So, in a perverse way, FDR's decision to fortify the island lured the Japanese into a trap. Not the trap he had envisioned, but a trap nonetheless.
Like Guadalcanal, a base that could not be adequately or economically supported.
I'm not sure I agree and he doesn't give any deployment data to back up that assumption but I'm not sure that the US was sending 85% of everything vs Germany. If so for my own curiosity I'd like to see the data to support that as I always assumed that most of production when to the ETO/MTO first.
When Japan lost the Battle of Midway in June 1942, Japan went from a superiority in aircraft carriers to a slight disadvantage. (The USN promptly added Saratoga and Wasp to the Enterprise and Hornet.) Nevertheless, Japan kept up construction work on a new airfield on Guadalcanal, and the island was still garrisoned mostly with construction troops. In essence, in Guadalcanal Japan created a target too juicy to resist.
My thinking is that by creating the juicy target, and not fortifying it enough to stop or even delay an American invasion, Japan created a battleground where it was constantly at a disadvantage. Setting aside the political issues regarding giving up ground, would Japan have been better off letting the US have Guadalcanal, and setting up a defensive position further up the Solomon Islands chain, closer to its base at Rabaul? The upsides to Japan would be a shorter journey for ships and planes from Japanese bases. An obvious downside would be that, with Guadalcanal undefended, there was nothing to stop the US from turning it into a major base more quickly than what historically occurred.