Short answers, pulled from the story.
The Pacific War is most widely accepted to have begun in 1941, when Japan attacked the United States and British Empire, though war between Japan and China had started in 1937 with hostilities dating back to Japan's invasion of Manchuria in 1931. The war ended with Japan's unconditional surrender on the 15th of August 1945, and the formal surrender document was signed on the 2nd of September 1945.
The Battle of Midway, fought in June 1942, was a decisive US naval victory in which American dive bombers destroyed four Japanese fleet carriers, Soryū, Kaga, Akagi, and Hiryū, the core of Japan's carrier striking force. After Midway, Japan experienced great difficulty replacing its losses in ships and experienced aircrew, while the United States produced ever-increasing numbers of both, permanently shifting the balance of naval power in the Pacific.
Casualties in the Pacific War were enormous across all fronts. By the eve of Pearl Harbor alone, some 7,500,000 Chinese soldiers and civilians had died in the Japanese invasion. On Guadalcanal, over 25,000 Japanese soldiers died compared to just over 7,000 Americans. Japan's Three Alls Policy killed at least 2.7 million civilians, and Japan's biological warfare operations in China killed an estimated 250,000 civilians.
American submarines were critical to defeating Japan, accounting for 56% of Japanese merchant ships sunk and 28% of all Japanese warships destroyed, including at least eight carriers. In total, Allied submarines destroyed approximately 1,200 Japanese merchant ships, around five million tons of shipping, which cut off nearly all the oil imports essential to Japan's war effort. By early 1945, the remaining IJN fleet was virtually stranded in port due to fuel shortages caused by the submarine campaign.
Japan attacked Pearl Harbor on the 7th of December 1941 to neutralize the US Pacific Fleet before seizing economic resources in the Dutch East Indies and Malaya, particularly oil, in order to break the Western embargo that threatened to halt Japan's economy. Japanese planners believed that a sudden, massive blow to American naval power would force the United States to agree to a negotiated peace that would recognize Japanese control of Asia.
Guadalcanal, fought from August 1942 to February 1943, was the first major Allied offensive in the Pacific and the campaign that stripped Japan of the strategic initiative. Over 25,000 Japanese soldiers died there, roughly 15,000 from starvation, compared to just over 7,000 Americans. Japan's inability to replace its losses in experienced aircrew and transports, combined with the campaign's drain on resources, also forced Japan to cancel its planned ground offensive against the Chinese Nationalist government in the Sichuan Basin.