Probably the most famous action in the Pacific War was the attack on Pearl Harbor on 7 December 1941. However, the attack on this port had the intention of preventing the American navy from hindering the Japanese invasions of Guam and Wake Island, the main goals, and which would be carried out almost concurrently.

Before the attack on Pearl Harbor, the Japanese controlled the following chains of islands to the Southeast of their country: Bonin, Marianas, Carolinas, and Marshall. Guam was inserted between the Marianas and the Carolinas, and Wake was almost adjacent to the Marshalls. These two American bases had to be eliminated to protect the right flank of the projected Japanese expansion.

The Japanese bombarded Guam a few hours after Pearl Harbor had been attacked and invaded the Island on 10 December (the Americans surrendered the same day). Almost simultaneously, they successfully bombarded Wake, destroying 8 Wildcats on the ground. They invaded the atoll on 11 December with a small force but were repulsed losing 2 destroyers. The Japanese returned on 23 December with a larger force, and this time they captured the island within hours.

The surprise attack on Pearl Harbor was not only extremely successful from the point of view of the destruction of capital ships in port (the most successful attack against a navy during the whole war in any theater) but the attacks on the US airfields were quite effective as well. The 188 aircraft destroyed on the ground and in the air in a single day is one of the largest recorded losses by any air force during WW2 during a single day of operations and in the case of the Americans, it was probably only exceeded by Operation Bodenplatte (Luftwaffe attack against USAAF and RAF airfields near the end of the war)

At the time of Pearl Harbor, Japan also controlled (from South to North in an almost straight line): Formosa (Taiwan today), the Ryukyu Islands, Japan, and the Kurile Islands. Since they had invaded China, French Indochina, and conquered Korea, they had indisputable control over the East China Sea and the Sea of Japan
This prevented the Soviets from access to the Pacific and left the Chinese with an entry only through the South China Sea. To complete the control of this sea and attain unrestricted access to the Indian Ocean it would be necessary to invade the Philippines, Thailand, Burma, Malaya, Hong Kong, and the Dutch East Indies and this is exactly with the Japanese imperial forces did starting with the attack on Malaya, and Hong Kong on 8 December 1941