The Battle of Midway was important to the history of the United States because it represented the main turning point in World War II in the Pacific.
Beginning with the Japanese attacks on Pearl Harbor and many other places in December of 1941, the US was losing the war all over the Pacific. The Japanese were pushing the boundaries of their empire farther and farther. They were threatening to cut off Australia in the south and had plans to take the island of Midway, which was about 1300 miles from Oahu. Midway was important to the US as a submarine base that extended the range of American submarines. It allowed the submarines to refuel and resupply without having to come all the way back to Pearl Harbor. The Japanese believed that the US would defend Midway tenaciously. They believed that attacking Midway would cause the US to send out its entire Pacific fleet. This would allow Japan to destroy that fleet and seriously reduce American power.
As it turned out, the US won this battle. Through a combination of luck and tenacity (along with the advantage of knowing that the Japanese were coming), the US managed to sink four Japanese aircraft carriers. The US also shot down many Japanese planes and killed many of their pilots. By doing so, the US weakened the Japanese profoundly. After this, the Japanese were no longer able to expand into the Pacific. Instead, they were placed on the defensive as the US built up more and more power by using its industrial might. This inexorably led to the defeat of Japan.
Thus, the Battle of Midway is important in US history because it served as a major turning point in WWII in the Pacific.
Midway was the turning point of the Pacific Campaign in World War II, but can actually be looked at as the culmination of a series of events.
It started with the bombing of Pearl Harbor on Dec. 7, 1941. In the aftermath of the devastating attack, FDR wanted to hit back, by striking the Japanese homeland. This led to the daring Doolittle Raid on April 18, 1942.
Despite the success of the Doolittle raid, US Naval Command feared that the success could "boomerang" on them, making Japan even more aggressive. They were right. The Doolittle raid convinced Japanese leaders to accelerate plans for an operation intended to capture Port Moresby in the Dutch East Indies (Indonesia) and gain control of the Oil and Rubber resources. Naval Intel figured out that there was something going down in the Coral Sea, and the USS Lexington and USS Yorktown were dispatched to investigate. The ensuing battle raged from May 3-May 8. The USS Lexington was sunk, the Yorktown suffered significant damage, but survived. The Japanese lost a mini-carrier, and one of 6 "front-line" carriers suffered significant damage. A number of smaller vessels, escorting the carriers on each side were also lost. Although a tactical draw it was a strategic victory for the US as the advance on Moresby was called off.
About the same time as Coral Sea, intel began hearing chatter about an Objective AF, and by May 9 confirmed that AF was Midway. Based on further intel, command began to suspect that the next Japanese move was Midway Island. The US Carriers were dispatched to a point NE of Midway under the command of Rear Admiral Ray Spruance.
Between May 31, and June 3 US and Japanese carriers exchanged Aerial attacks that resulted in the destruction of 4 Japanese Carriers, fatal damage to the USS Yorktown, and significant damage to the US Airfields on Midway Island. This left the US and Japan on nearly equal Naval footing, and the US was able to recover faster with superior industrial capacity. Japan was on the defensive from that point until the end of the war in Aug. 1945.