The short version: Aircraft carriers were recognized by 1941 as capital ships that would play an important role in the fleet. However, the technology and tactics that underpinned naval aviation was rapidly progressing before and during World War II. What was capable for aircraft carriers in 1945 was vastly different than in 1941, which was vastly different than in 1937. Naval aviation was not yet a mature technology like it mostly is today. As a simplistic comparison, think of how different social media was in 2006 compared to 2016. With this in mind, there were certainly some civilian and military leaders who did not fully grasp the importance of aircraft carriers in 1941. Others, however, were more up-to-date. You can find disagreement about the objectives of the Japanese attack on Pearl Harbor, but aircraft carriers were certainly a high priority. Aircraft carriers were already coming into their own in the late 1930s and early 1940s. The outbreak of war made that reality clear to everyone relatively quickly if it wasn't already apparent.

The longer answer...

Japan's objective during the attack on Pearl Harbor was to destroy the American fleet in the Pacific to prevent it from carrying out a counterattack as it consolidated its gains throughout the rest of the Pacific. This was a strategic objective but it also had a political aim; Japanese military leaders believed they needed about two years to capture and consolidate their new empire in the Pacific. This two-year period would allow them to begin extracting resources from these areas to expand the Japanese economy and reinforce these areas before the Americans could mount a campaign to retake these areas. They hoped that the Americans would decide to seek terms rather than undergo a two-year military buildup and then fight a bloody war throughout the Pacific. The Pearl Harbor attack was meant to destroy the fleet, but also to break the will of the Americans for a counterattack.

Because of this, the American battleships at Pearl Harbor were a vital target. The battleship in 1941 remained the ultimate symbol of military power in the world. Battleships were the most expensive piece of military equipment at the time, took years to build, were crewed by thousands of men, and wielded massive destructive power. Sinking a battleship was not just destroying an enemy weapon, but destroying a symbol of that country's power. The United States Navy had never lost a battleship to enemy attack before (you can quibble about whether USS Maine was a battleship, or whether it was lost to enemy attack), so the destruction of one or several battleships would be considered shocking -- and it was when the attack occurred. Alan Zimm, in his excellent book Attack on Pearl Harbor: Strategy, Combat, Myths, Deceptions, puts forward his belief that the Japanese objective was to sink battleships at Pearl Harbor rather than aircraft carriers.

However, Japanese planning clearly showed that aircraft carriers were also a high priority. The Japanese priorities for the attack were divided into tiers. The first tier was battleships and aircraft carriers. The second tier was cruisers, destroyers, and other warships. Shore facilities, naval auxiliaries, and other ships were below that. In fact, aircraft carriers were singled out in at least one way: They were targeted in both the first and second waves.

The first wave of Japanese planes included 183 planes (189 were planned, but six failed to launch or turned back because of various technical failures). There were 43 fighters meant to engage American aircraft or destroy aircraft on the ground, 51 dive bombers meant to attack American airfields, 49 horizontal bombers meant to target battleships, and 40 torpedo bombers meant to target battleships and aircraft carriers. The second wave included 171 planes (175 were planned), with 35 fighters, 54 horizontal bombers meant to target the airfields, and 78 dive bombers meant to target American ships. The dive bombers were instructed not to target the battleships, because their bombs couldn't pierce the armor of the battleships. However, they were instructed to target the aircraft carriers. Even if the torpedo attacks in the first wave had sunk the aircraft carriers, the dive bombers were instructed to bomb the hulks of the carriers to prevent them from being refloated and returned to service. Clearly, aircraft carriers were a high priority.

The next part of this is explaining the development of naval aviation in the years leading to 1941.

Early concepts for carrier operations stipulated that the aircraft carriers would support the battleships. The obvious application was to use the airplanes launched from aircraft carriers to scout ahead of the fleet, report the position of enemy ships, and spot for naval artillery (ie, observe where the shells were landing relative to the target to provide adjustments for the guns). This sounds foolish in hindsight, but it was probably the best use for the earliest aircraft carrier designs in the 1920s. Early aircraft were relatively slow and did not have the powerful engines necessary to carry the bombs or torpedoes that could damage enemy battleships. A shell from an interwar battleship would weigh in the neighborhood of 2,000 pounds, and battleships were designed to absorb multiple hits from these shells and keep fighting. It would take some time for aircraft to carry the weapons that could deliver similar destructive power.

Still, wise admirals quickly realized the importance of naval aviation and aircraft carriers. The ability to find and track the enemy fleet was a huge advantage. In interwar wargames and exercises, aircraft carriers quickly became the first ships targeted for enemy attacks. Aircraft carriers did not usually carry the heavy armor that battleships did (the Royal Navy, notably, did build armored flight decks on its carriers) and were filled with all kinds of aviation fuel, oil, and weapons that could easily destroy the ship if it was attacked. Carriers were valuable but fragile. Interwar naval arms treaties limited the number and size of aircraft carriers that could be constructed, but American naval officers correctly predicted that many more carriers would be needed in the event of an actual conflict. The design of aircraft carriers themselves also improved. USS Lexington (CV-2) and USS Saratoga (CV-3) had been laid down as battlecruisers and converted to aircraft carriers as they were constructed. USS Ranger (CV-4) was purpose-built as a carrier, but was smaller and included flaws that would be remedied in later designs. The USS Yorktown (CV-5) and USS Enterprise (CV-6) were much-improved as a result of the lessons learned from earlier carriers, and the Essex class (CV-9 and up) built mostly during the war were further improvements. I am less familiar with the Royal Navy’s carrier designs, but the Japanese Navy went through similar iterations of building carriers on hulls converted from other types and applying lessons to later iterations.

The design and capabilities of aircraft also improved considerably. A torpedo might weigh up to 2,000 pounds, and the armor-piercing bombs that the Japanese horizontal bombers carried to attack Pearl Harbor (such as the one that destroyed USS Arizona) also weighed almost 2,000 pounds. In fact, the armor-piercing bombs at Pearl Harbor were converted from shells built for Japanese battleships. Planes got faster, and slower planes would have been easier to shoot down. Aircraft range improved, meaning it was easier for carriers to operate a safe distance away from enemy ships -- other than enemy carriers. Radio communication for aircraft improved. All of these developments made the aircraft launched from aircraft carriers much more dangerous by 1941 than they had been a few years before. The standard U.S. Navy fighter in 1941, the F4F, was the first carrier-based monoplane put into production for the Navy. Its predecessor, the F3F, had been a biplane. Improvements continued during the war; the Navy replaced the F4F with the F6F mid-war. The SBD Dauntless dive bomber arrived in late 1940 or early 1941; it was replaced with the Helldiver in 1943. The Devastator torpedo bomber was close to being replaced by the Avenger in 1941. If the war had continued into 1946, even that generation of aircraft might have been replaced. The Japanese were making similar progress but lacked the industrial capacity to produce new aircraft in numbers to keep pace with the United States. This is how fast the technology was progressing before and during the war.

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