On a general level, the Royal Navy were well aware of the capabilities of aircraft, and the threat they posed to warships. What the commanders on the ground lacked was a specific understanding of the threat posed by Japanese aircraft - in particular, the ranges of Japanese naval strike aircraft. The sinking of Force Z was not a doctrinal failure, but an intelligence and staff one.

The Royal Navy had spent most of the 1920s and 1930s grappling with the new abilities and dangers that aircraft raised. I've previously discussed the RN's understanding of air defence, as well as its plans to use aircraft in a naval battle. These answers cover the RN's interwar planning; the lessons of the first two years of the war were seen as generally confirming this thinking. The RN had faced countless air attacks in the European theatre. Off Norway, it had suffered heavy air attacks, which, amongst other damage, sank the destroyer Gurkha and cruiser Curlew and heavily damaged the cruiser Suffolk. The Battle of Britain had opened with a concerted attack against British convoys through the English Channel. This took a heavy toll on merchant shipping, and sank a number of destroyers. As the Italians entered the war, the RN found itself fighting under an enemy air umbrella that vastly outnumbered the opposition the Fleet Air Arm and RAF could put up. British ships in the Mediterranean, especially those operating in the Central or Northern Mediterranean, saw frequent air attacks. These did heavy damage to a number of capital ships and carriers, and sank many cruisers and smaller ships. The RN had also had experience in using aircraft to attack. FAA divebombers had sunk the German cruiser Konigsberg at Bergen during the Norweigan Campaign. FAA torpedo bombers had damaged the Bismarck, and elements of the Italian fleet at Matapan, allowing British surface forces to catch them. The FAA had carried out a raid on the Italian fleet base at Taranto, sinking one battleship and heavily damaging two more. In combination with the RAF, the FAA were carrying out an effective campaign of attacks against Axis convoys to North Africa. This experience, though, tended to confirm the RN's pre-war thinking. Attacks by level bombers were frightening, but did little damage as their bombs were easily avoided. Torpedo bombers were much more threatening, but were easily dissuaded by AA fire and could not sink a capital ship unless massed. Dive bombers were highly dangerous, but only to lighter ships, as the armour of capital ships could prevent serious damage. Fighters and AA were needed to protect ships against attack; ideally, the fighters would come from carriers. To this end, new carriers were constructed, and the AA outifts of ships were increased by fitting new weapons like the 20mm Oerlikon gun.

It should be clear that the RN did not take the threat from aircraft lightly, given its thinking in the 1930s and its war experience. Given this, why was Vice Admiral Tom Phillips so blase about the threat from Japanese aircraft? The most likely answer is that he did not expect a seriously threatening air attack in the South China Sea. This expectation was the result of a number of factors. British Naval Intelligence had misjudged the capabilities of the IJN and its aircraft. It had also not identified the fact that it had moved naval strike aircraft forwards to airfields where they could reach deep into the South China Sea; or if it had correctly identified these facts, they were not properly passed forwards to Phillips.

Determining exactly what the RN knew about the capabilities of Japanese aircraft is difficult. The British gathered information about the Japanese through a wide variety of methods. British naval, air and military attaches picked up information through visits to airfields, dockyards and factories. They may also have operated spy networks, as did MI6. Submarines photographed fleet manouvres and docks. British codebreakers regularly read Japanese signals and messages. To avoid duplication of effort, the intelligence these disparate methods produced were fed into an organisation called the Far East Combined Bureau (FECB). The FECB was a tri-service operation which combined information produced by the intelligence units of each service into one. While it was quite effective, there were a number of problems. It had a perceived bias towards naval issues. It had originally been formed in Hong Kong, but the encroaching possibility of war forced it to move to Singapore in early 1939. This meant that the RAF and Army carried out their own independent investigations of the Japanese threat.

The FECB had made varying assessments of Japanese aircraft performance. The performance figures it circulated in the 1930s were generally quite far off. David Hobbs cites a 1938 Admiralty intelligence publication which gives a maximum range for the G3M Nell bomber (one of the aircraft types that attacked Force Z) of 732 nautical miles. The actual range of the aircraft was 2,365 nm. However, by mid-1941, the performance figures were far more accurate - but it is unclear how far these were circulated beyond the intelligence community. There are indications that the RAF understood the figures, but knowledge of them in the RN seem to be lacking. The performance of Japanese aircraft was not the only issue; numbers, doctrine and organisation were also important. The FECB made highly accurate measurements of aircraft numbers within the IJAAF and IJNAF (the air forces of the Japanese Army and Navy respectively). Estimates of IJNAF numbers in December 1941 put the front-line strength of the force at 1561 aircraft, of which 547 were ship-borne. This was an overestimate compared to the real numbers, which were 1477 aircraft, of which 646 were ship-borne. While the FECB had got the numbers right, they missed key factors of Japanese organisation. The IJNAF had formed a joint striking force in the 11th Air Fleet. This integrated scouting and strike into a single organisation, lowering reaction times and increasing the cohesion of strikes. It also made it easier to operate alongside surface ships and aircraft carriers, though this would not be a factor with Force Z. The British had completely missed this, and assumed that the Japanese would operate as the RAF and Fleet Air Arm did. The FECB had also identified Japanese works to expand airbases around Saigon (and elsewhere in southern Indochina), but had not fully informed the RN that torpedo bombers could operate from these bases in December 1941.

Alongside these technical factors, the FECB had to assess Japanese fighting efficiency, a more nebulous term that describes a combination of doctrine, morale and other human factors. The assessment prevailing in 1941 was that the IJNAF was comparable in ability to the Italian Air Force. It's hard to know how the RN interpreted this. The general assumption was that the Italians were less capable than the British. The Navy, however, had seen frequent successful attacks by Italian aircraft. These had led British officers who had experienced these attacks, such as Cunningham or Somerville, to gain a considerable amount of respect for the Italians. Cunningham's memoirs state that 'Italian high level bombing was the best I have ever seen, far better than the German', and that Italian reconnaissance 'seldom failed to find and report our ships'. However, it is unclear whether or not Phillips had internalised the meaning of these reports; he may have hewed closer to the pre-war understanding, leading to a relaxed attitude towards the IJNAF.

When Phillips arrived in Singapore, he did not receive briefings from FECB. Nor, it appears, did his staff. However, he does seem to have had some knowledge derived from it. His planning included a worst-case situation where Japanese aircraft were based at Ca Mau (~150 miles closer than Saigon), which appears to have been taken from FECB advice. Phillips did seek advice from the RAF. His chief of staff, Rear Admiral Palliser, contacted Air Marshal Brooke-Popham to seek advice on the air threat before Force Z sailed. Brooke-Popham's advice was that the IJNAF could put 50-60 bombers into the air, some five hours after Force Z was spotted. The admiral's staff combined this knowledge with their experiences in the Mediterranean. These indicated that strikes were unlikely to have a maximum range of more than 300 nautical miles. They also assumed that the Japanese bombers would make hastily organised strikes, and thus were more likely to have bombs than torpedoes. Little attempt was made to seek deeper intelligence, either from the RAF or from the FECB. These failures meant that Phillips likely underestimated the threat. He steamed too far north - but did avoid attacking Singora, as this would put him well within his believed range for Japanese strikes and extended the time he spent within range in daylight. He failed to withdraw sufficiently quickly, leaving the force exposed. Wanting to stay undetected and keep radio silence, he did not call for air support or cover. However, it is possible that Phillips felt that he had no choice but to attack, despite the air threat. Some of his actions during the run-up to the start of war indicate an appreciation of the threat from Japanese aircraft. He strongly pushed for the deployment of Hurricanes to Singapore, and rejected Manila as a base for Force Z, as it was too close to Japanese air bases on Taiwan. Singapore's main defence was the fleet based there; if it did not act, then Singapore might fall. I consider it more likely, though, that there was a failure of staff work; the lack of communication with FECB meant that the RN underestimated the Japanese air threat.