1. Underestimating Japanese Capabilities: The US Navy believed the Japanese lacked the fuel and logistics to launch a large-scale attack across the Pacific. This misjudgment was partly due to the assumption that Japan would prioritize securing resources in Southeast Asia rather than attacking a well-defended base like Pearl Harbor.
2. Signals Intelligence (SIGINT) Breakdown: The US had broken the Japanese diplomatic code (known as "Purple Code") and intercepted some encrypted messages that hinted at a potential attack. However, cryptographers failed to decipher these messages in time or misjudged their importance, leading to a lack of specific warnings.
3. Lack of Radar and Aircraft Identification: The US military at Pearl Harbor lacked adequate radar systems and aircraft recognition protocols. The radars were turned off during the morning of December 7, and incoming Japanese aircraft were mistaken for American B-17 bombers conducting a routine training exercise.
Trust in Diplomatic Negotiations:
1. Ongoing Talks: The US and Japan were engaged in diplomatic negotiations in an attempt to resolve tensions in the Pacific. This gave rise to a sense of complacency and optimism that tensions would be de-escalated through diplomacy rather than military action.
2. Misreading Japanese Intentions: US intelligence may have misinterpreted Japan's military preparations as defensive measures, particularly in light of the concurrent conflicts in Europe. This misreading prevented a timely response to the developing threat.
Organizational Factors:
1. Communication Issues: There were communication breakdowns between different branches of the US military (Army, Navy, Air Corps) and even within the naval command structure at Pearl Harbor. Vital intelligence did not reach the appropriate decision-makers in a timely manner.
2. Chain of Command: The decision-making process within the US military was slow and hierarchical, particularly in an era when communication technologies were less advanced. Orders from Washington, DC, took time to reach the commanders in Hawaii.
3. Peacetime Mindset: The prevailing mindset in the US military was focused on potential conflicts in the Atlantic theater, given the ongoing events in Europe. This diverted attention from the possibility of a surprise attack on Pearl Harbor.
In summary, a combination of intelligence failures, trust in diplomatic negotiations, organizational shortcomings, and a lack of preparedness contributed to the US Pacific Fleet being caught off guard by the attack on Pearl Harbor.