In the years after World War Two, and indeed, even today, there has been much speculation on how an early warning of an impending Japanese attack on Pearl Harbor would have affected the outcome of the raid on that fateful 7th December 1941, when the US Navy Pacific Fleet lost all eight of its battleships. These have ranged from an optimistic prediction of a complete defeat of the Japanese force, to more gloomy suggestions that the Americans might not just have lost their battleships, but their aircraft carriers as well.
Any such speculation must, however, depend on how early a warning Pearl Harbor received. A 24-hour or even 48-hour warning may not have been enough to make a difference. Given the lack of American air superiority (neither the USAAF nor the US Navy possessed a fighter aircraft at that stage of the war that could match Japan’s legendary Mitsubishi A6M Zero), merely getting the battleships out of harbour would not have been sufficient as they would have been sunk in deep water where they would have been unsalvageable (unlike those sunk at their moorings inside Pearl Harbour; and six of which were later raised and returned to service). If the American aircraft carriers, which were out at sea on exercises at the time of the raid, had been brought back to fight they might have been sunk too. A warning of a week or two might have enabled the US Navy to simply move their capital ships out of range, or perhaps position the battleships for a night attack on the Japanese fleet. Without the necessary air superiority, however, Pearl Harbor could not have been defended, regardless of the warning.
A good understanding of alternative possibilities is to examine Admiral Chuichi Nagumo’s Indian Ocean Raid, three months later; specifically, the attack on the British Eastern Fleet and Ceylon (now Sri Lanka), in the first week of April 1942.
So what were the similarities? Nagumo had nearly the same force he attacked Hawaii with, the 1st Air Fleet, and five of six flattops he had used — the 5th Carrier Division, the Akagi, Shokaku, and Zuikaku, and the 2nd Carrier Division, with the Hiryu and Shoryu. With them were the four Kongo-Class battleships and the two Tone-Class cruisers. He was attacking an island, albeit one with two harbours instead of one — Colombo and China Bay, the latter being a superior deep-water port to Pearl Harbor — on the two opposite sides of the island.
And what was different? The British Eastern Fleet, under Admiral Sir James Somerville, was much smaller than the US Pacific Fleet, consisting of two modern carriers, Formidable and Indomitable, and the outdated Hermes, the modernised battleship Warspite, and four old Revenge-Class battleships. Somerville also had a few submarines. One major differing factor between Ceylon and Pearl Harbor was that the British had been expecting Nagumo to enter the Indian Ocean once he had sailed away from Hawaii, and had placed three RAF squadrons of Hawker Hurricanes on Ceylon — two at Colombo, and one at China Bay. Unlike the American P-40 Warhawks, the Hurricane was a match for the Japanese Mitsubishi A6M Zero.
The British expected Nagumo to sail from Staring Bay in Sulawesi on 21st March and to attack Ceylon on April 1st. British naval intelligence also believed that Nagumo only had two carriers with him. Nagumo in fact sailed a few days later, on the 26th, with all five carriers. Somerville put to sea on the 30th and placed his fleet in a patrol line south of Ceylon. When air reconnaissance had failed to detect the Japanese by 2nd April, Somerville pulled most of his ships back to the Maldives to refuel, and sent a few of the older ships back to Ceylon, including the carrier Hermes.
Twelve hours before his intended air attack, Nagumo’s fleet was spotted on the afternoon of April 4th, 360 nautical miles southeast of Ceylon, by a Consolidated PBY Catalina flying boat of the Royal Canadian Air Force’s 413 Squadron, based at Koggala, on the southern coast of Ceylon. Though the Cat crew got off a radio warning, they were shot down before they could report the Japanese fleet’s size. Somerville immediately launched his faster ships east towards the Japanese.
At dawn on April 5th, Easter Sunday, Nagumo launched his attack without any air reconnaissance of the target. A force of 91 Japanese bombers and 26 fighters hit Colombo at 0800, but found the harbour mostly empty — Admiral Geoffrey Layton, commanding the Ceylon defences had, when he heard the Catalina’s warning, sent Somerville’s older ships back out to sea to meet him. Eighteen Japanese planes were shot down, but the RAF lost 27 planes.
Nagumo recovered his first wave of planes, cancelled the second, and launched a new attack on the older British ships escaping Colombo, sinking the cruisers Cornwall and Dorsetshire, which went down with over 400 men, all told.
While both opposing fleets spent the next two days trying to find each other with aerial reconnaissance, neither side succeeded. By the 8th, Somerville had realised his fleet was heavily outsized by Nagumo’s, and pulled back to the Maldives to refuel. Nagumo turned east and then north to attack China Bay on the northeast coast of Ceylon.
On the afternoon of the 8th, another Catalina spotted Nagumo’s fleet 400 miles off Ceylon’s eastern coast and, that night, China Bay was cleared of ships, with the carrier Hermes sailing down the Ceylonese coast with a single escort. The Japanese attacked China Bay on the morning of the 9th with a strength similar to the Colombo raid, and were met by 22 British aircraft. The harbour facilities were badly damaged, but no ships were in the port. The Japanese then attacked Allied shipping all along the eastern and northeastern coast of Ceylon, catching Hermes and her escort (the carrier had left its aircraft ashore) off Batticaloa and sinking both. Simultaneously, nine Bristol Blenheim bombers of 11 Squadron attacked Nagumo’s fleet, arriving completely undetected over Akagi at 11,000ft, attacked immediately, and missed completely. Four Blenheims were shot down. This was, however, the first time, a Japanese carrier force had been attacked by aircraft, and its defences were easily penetrated.
In four days of fighting, the Japanese heavily damaged both harbours, sank a carrier and two cruisers, destroyed a third of the RAF’s fighter defences, and all of its strike aircraft. They also sank 23 merchant ships, totaling over 112,000 tons. They lost less than 20 aircraft, and no ships. However, they completely missed the British Eastern Fleet, which was the main target of the raid.
Having missed Somerville’s fleet, Nagumo turned back east, needing rearming and replenishment, and was back in the Pacific by June. In return, the British realised the RAF wasn’t strong enough to defend the Ceylonese ports, and that the Eastern Fleet wasn’t strong enough to meet the Japanese, and pulled it back to Kenya until it could be reinforced and the island more heavily defended. While the Eastern fleet continued to patrol east to Ceylon and the central Indian Ocean, Nagumo had effectively pushed the Royal Navy out of the eastern Indian Ocean. Britain didn’t envision being able to gather the necessary forces to return the Eastern Fleet to Ceylon until September. These plans too were eventually downgraded after June when the Battle of Midway reduced the threat of Japanese operations in the Indian Ocean.
If this battle were to be used to build a hypothesis for Pearl Harbor, it’s possible to note a few Japanese weaknesses that might, perhaps, have been to the US Navy’s advantage. Admiral Chuichi Nagumo made very poor use of aerial reconnaissance; failing to detect that the British Eastern Fleet had left port before attacking and, even after realising it had left Colombo, continuing to press his attack on China Bay without concentrating on locating the British fleet. If the US had received a couple of day’s warning of the attack on Pearl Harbor, early enough to get their battleships out of port, Nagumo may not have gone looking for them, but instead pressed on with attacking Pearl.
Additionally, the Blenheim attack on Akagi revealed that the Japanese air arm was incapable of defending the fleet while simultaneously launching attacks. If the American carriers had been within range of Nagumo off Hawaii, perhaps a successful counterattack could have had some success. The American weakness in fighter aircraft makes this an arguable proposition, though
If, however, the warning had come a week earlier, the US Navy might have deployed in a blocking position, and Nagumo would have found them without having to look. Whether the three American carriers could have driven off the six Japanese carriers without a superior fighter to the Zero is a good question. The result might have been the destruction of the Pacific Fleet’s air strength.
Perhaps the best use of a warning might have been — both for the British Eastern Fleet and the US Pacific Fleet — to simply have withdrawn out of Japanese reach. Somerville’s attacks had very little effect in preventing the Japanese attacks on Ceylon’s ports. His most effective action was simply putting to sea and placing his ships out of initial Japanese reach.
However, it is probably unlikely that the US Navy would have pulled the Pacific Fleet out of Japanese range. The Royal Navy had been in action for almost three years and understood that discretion was sometimes the better part of valour; the Americans would have been more averse to retreat, given that this was their first hostile engagement of the Second World War. So perhaps it was better that there was no warning.
This post is an expansion of an answer provided for Quora two years ago.