For those unaware of the Battle of Savo Island, I'll try to set the stage. The video is good, and you can skip ahead to about 31:00 or 36:20 to get to the battle itself.

After the Allies repulsed Japanese offensives at the Battle of the Coral Sea and the Battle of Midway in May/June 1942, the decision was made that it was time to go on the offensive in the Pacific. The target was the Solomon Islands, at the far southwestern edge of the Japanese empire. Guadalcanal would soon become the most famous of these islands. It was called "Operation Watchtower." American commanders made the decision to adopt an aggressive timetable; the invasion date was pushed up from fall 1942 to August 7. It was a contentious decision, not the least because it was the first major amphibious operation for the US Navy/Marine Corps in World War II, forward bases in the Pacific weren't yet built out, and the American aircraft carrier fleet still hadn't recovered from the loss of Lexington and Yorktown at Coral Sea and Midway, respectively. There were 4 American aircraft carriers in the Pacific: Saratoga, Enterprise, and Wasp were all sent to participate in Watchtower while Hornet remained at Pearl Harbor. With this in mind, there were various priorities for Watchtower. Not necessarily in order...

1. Prevent the completion of a Japanese airfield on Guadalcanal, capture other key Japanese bases in the area (Tulagi, etc.)

2. Surprise the Japanese in the region with the aggressive timetable.

3. Preserve American carrier strength.

The Japanese were surprised and the invasion on August 7 went off relatively smoothly. Guadalcanal, Tulagi, and Gavutu were all captured in short order. However, one of the points of contention during the planning of the operation would be how long Admiral Frank Jack Fletcher (a hero at Coral Sea) would keep his aircraft carriers in the area to provide air cover while the amphibious fleet unloaded supplies for the Marines. Fletcher ultimately promised to remain in the area for 48 hours after the invasion; shorter than Admiral Richmond Kelly Turner (of the amphibious fleet) and USMC General Alexander Vandegrift wanted. Turner and Vandegrift believed that wasn't enough time to offload the supplies, putting the Marines at risk of being cut off without ammunition, food, medicine, heavy equipment, etc. But protecting the carriers was still important. The Japanese had several airfields in the area that could (and did) attack the invasion fleet, and Fletcher became worried that his carriers were vulnerable to enemy air attacks. Fletcher ultimately withdrew in the evening of August 8, ahead of his 48-hour window. During the night of August 8/9, a Japanese surface force of cruisers and destroyers sailed down "the slot" in the Solomons and caught the remaining American naval forces guarding the transports by surprise.

In nighttime surface combat, the Japanese sank 3 American heavy cruisers, and a fourth Australian cruiser was scuttled when the remaining forces had to withdraw. Another destroyer cruiser was damaged and 2 destroyers were damaged. More than 1,000 Allied sailors were killed. The Japanese had 3 cruisers damaged, but all 3 sailed out under their own power. Not counting Pearl Harbor, it's probably the worst defeat for the US Navy in World War II and perhaps the worst in its history. The Marines were left to slog it out on Guadalcanal with limited supplies, and the Navy was dragged into a protracted campaign to try and maintain local supremacy in the region for several months.

Now, to answer your question.

Maybe the most comprehensive report on Savo Island was this 1950 study of the battle by the US Naval War College lists 26 "Lessons Learned" from the battle.

The video does a good job of explaining many of the failures in the command structure, communications, and overall control of the battlespace. I'll touch on that briefly for those who didn't watch the video. The overall command of the units involved in Watchtower was divided between the Southwest Pacific Commander (COMSOWESPAC), MacArthur, and the South Pacific Commander (COMSOPAC), Admiral Ghormley. This led to a situation where a search aircraft under MacArthur's command didn't relay a report to Ghormley's command for hours because of the convoluted line of communications it had to pass through. The command of the cruiser force off Savo Island was also plagued by poor communication. Even during the battle, the commanders of the cruisers did a poor job of communicating with one another or their superiors. Some of the officers also appeared to be waiting for instructions from Admiral Crutchley, who had previously departed for a conference without informing all of the forces under his command. The overall command in the region was muddled, but there was basically no overall Allied commander at Savo Island as the cruisers fought mostly independently.

Some "lessons learned" were known at the time, but decisions that had to be made based on the tactical and strategical circumstances. The Americans might have preferred to wait and launch an offensive that had more time to gather forces in the area, but the construction of the airfield on Guadalcanal posed a threat to shipping heading to the New Hebrides and other Allied holdings in the area. So the attack was sped up, even if the situation wasn't perfect. In later invasions, the American carrier task force would spend days or weeks in the area to destroy enemy airpower, then linger to assist the Marine assaults. But at the time, the United States didn't have dozens of carriers in the Pacific, and couldn't afford to lose what it had. Later invasions were given more time to offload supplies and greater air cover. The invasions in the Gilberts in late 1943 were supported by 6 large fleet carriers, 6 light carriers, and a handful of smaller escort carriers. The situation in August 1942 was just fundamentally different. In essence, the tactical situation for the invasion of the Solomons was less than ideal. But the strategic situation dictated that it was better to go ahead than to wait.

The American over-reliance on radar was noted. Quoting directly from the 1950 report:

So confident was CTG 62.6 as to the capabilities of his radars that he stated later that it should not have been possible for an enemy force to get inside Savo Island without being detected either visually or with radar by the screening destroyers BLUE and RALPH TALBOT. He particularly stressed radar. He stated that these screening destroyers were so stationed that radar with a surface range of but six miles would insure that nothing got by. He did not realize that the BLUE and RALPH TALBOT could be as much apart as twenty miles. Neither he nor his commanding officers appear to have been familiar with the limitations of the radar of those days or with its failure in the presence of land masses.

This is considered not so much a failure of the radar itself, but a failure on the part of the commanding officers to understand the limitations of radar at the time.

The success of the Japanese during night combat was certainly noted. The Allies didn't even have a specific night battle plan to lay down general expectations for how to fight (a situation exacerbated by the poor command structure and communication). According to the 1950 report, the Japanese expended about 1,867 rounds of gunfire with 159 to 223 hits, a success rate between 8.5 percent and 12 percent. The Allied forces fired 471 shells and registered 10 hits, slightly better than 2 percent. This was not the first or the last time the Allies faced the IJN in night combat, but it seems to have left the strongest impression. If there was one combat that led the Americans to realize they had underestimated the IJN's night fighting capabilities, this was it. The Allied ships in the battle had a minimum of training in night combat, and much of that training was several months old. However, the outcome was not that the Americans eschewed nighttime action with the Japanese, even in the Guadalcanal campaign. They just had to do a better job of preparing for it.

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