while the Marines had almost an old battleship per battalion (and definitely one per regiment) at Saipan, the Allies at Normandy had about one per division. Definitely the gun line should have been beefed up at Omaha and Gold Beach. The ships could have been found for that and should have been. There were also cruisers to spare as well.
On the one hand, a lot of that had to do with the density of the defense in the Marianas islands. Saipan isn't even 45 square miles. The Japanese had a garrison of 32,000 troops. Not only is there not a lot of room for maneuver, there's no room for the Japanese to retreat, either (not that they were generally inclined to retreat). Perhaps more importantly, Japan actually had a navy capable of interfering with the invasion of Saipan (at least until June 20), whereas Germany really did not - and a major motivation for FORAGER was, after all, to force the Combined Fleet to engage in a decisive battle in which it might be destroyed.
On the other hand, the massive resources dedicated to FORAGER (the Allied campaign to secure the main islands of the Marianas) at the very same time that OVERLORD (and DRAGOON, for that matter) - 128,000 troops, and the cream of the United States Navy - made a bit of a mockery of the principle of "Germany First." Yes, as it turned out, the United States had the capability to do both at the same time; but as it turned out, it had that capability because Allied deception efforts were so successful in misleading the Germans into thinking the invasion would come elsewhere. Had they deployed the bulk of the forces sitting in reserve in the Pas de Calais, every battleship and cruiser in Nimitz's vast inventory wouldn't have been too much. And while Allied intelligence of German deployments in Normandy was pretty good, they couldn't be completely certain of what was deployed there.
That said, delaying FORAGER until the autumn would have given the Japanese that much more time to fortify not only the Marianas more thoroughly, but plenty else besides in Japan's inner empire. Oil might be in short supply, but plenty of other things were not. Most of the defenses of Iwo Jima (and reinforcements) was put in place in the five months before the battle. Had the US moved on Iwo Jima immediately after securing the Marianas, rather than waiting until February, Iwo Jima would have been much easier and less costly to secure. One shudders to think what Saipan could have been like had Saito had another 4-6 months to fortify the island, even with submarines interdicting a lot of shipping from Japan.
