On 20 April, after the Combined Fleet held repeated exercises at the Naval University, all the ships participating in the surprise attack conducted the final comprehensive attack exercise in full accordance with the raid plan and aimed at the main battleships of the Combined Fleet assembled in Saeki Bay, as a test of pre-war training. Through these exercises, Yamamoto felt that the attack had achieved good results and had achieved the purpose of training, and the participating troops summed up the experience of the exercises, returned to their respective bases, and made final preparations for departure.

On 25 April, the captains and flight officers of all the participating ships discussed the key issues in the combat plan. On April 27, all the pilots made their missions clear on a sand table model of Pearl Harbor.

In order to know oneself and one's opponent, the Japanese army has put a lot of effort into collecting intelligence on the US military. Since May 1941, as many as 200 Japanese spies have been sent to Pearl Harbor to collect the weather, hydrology, and topography of Pearl Harbor, as well as the deployment of US military bases, aircraft, and ships. Special mention should be made of Takeo Yoshikawa of the Japanese Naval Intelligence Department, who, after eight months of training, came to Pearl Harbor in March 1941 as a counselor at the Japanese Consulate in Honolulu under the pseudonym of Masashi Morimura to conduct intelligence gathering. The information is then sent back to the Ministry of Foreign Affairs in a diplomatic code, and then forwarded to the Navy. The information he sent back grew from once a week at the beginning to once a day at the end, and provided the Japanese Navy with a great deal of valuable information, including: the anchorage and activities of US warships on different days, the types and numbers of US planes at Pearl Harbor, and the air defense facilities at Pearl Harbor.

In order to obtain the actual situation of the northern route, the Japanese army sent two groups of four officers in October, disguised as merchants or sailors, to Honolulu on ships passing through the northern route to inspect the meteorology, sea conditions, and shipping conditions of the northern route. In addition to sending spies to obtain intelligence, the Japanese military also stepped up surveillance of Hawaiian radio and telecommunications. As a result of these efforts, before the start of the war, the Japanese army had accurately grasped the US defense facilities at Pearl Harbor, the deployment of troops, the types and numbers of ships and aircraft, the situation of stationing and parking, and the pattern of US military activities.

In order to ensure the suddenness of the sneak attack, the Japanese army also adopted a series of strategic deception and camouflage. First, use peace talks as a cover. As early as February 1941, the Japanese side appointed Yoshizaburo Nomura, a retired admiral and a friend of US President Roosevelt, as its ambassador to the United States. During his tenure as ambassador to the United States, he held dozens of meetings with U.S. officials. In July last year, Prime Minister Konoe sent a letter to Roosevelt, saying that there was no problem between the two countries that could not be resolved through negotiations, and that he would not infringe on the interests of Britain and the United States in Southeast Asia. On 5 November last year, Japan's base camp also sent a recognized pro-US figure who was a consul in the United States and married an American to Saburo Lai Su, as a special envoy for peace, to the United States for the first round of talks.

Second, from July 1941, when the Japanese side began to make secret contacts with the Soviet Union, the Japanese Kwantung Army began to expand one after another, increasing its strength from 400,000 men in 11 divisions to 700,000 troops in 20 divisions. In October 41, a landing exercise was organized in Japan's inland sea, and military attachés of various countries in Japan were also invited to visit in order to attract the attention of various countries. In December, the regular mail ship "Tatsuda Maru" from Yokohama to Honolulu sailed as usual. For three days from December 5 to December 7, 3,000 cadets of the Etajima Naval Academy were organized to dress up as naval officers and sailors to tour the cityscape of Tokyo, creating a peaceful scene. During the days of carrier-based aircraft training, planes from other units were dispatched to the bases of the participating units to maintain the normal flight and communications of these bases. These cover-up measures can be described as painstaking.

Finally, confidentiality measures should be strengthened. For a long time, the plan was known only to Yamamoto and a very small number of high-ranking officers. The participating units were trained in Kagoshima and Saeki Bay in southern Japan, while they were assembled in Monoguan Bay, a island in the northern part of Japan. During the assembly of the assault formation, each ship chose a remote route away from the merchant shipping route and went to it in batches at different intervals. During the voyage, the transceivers of all ships were sealed with lead seals, strict radio silence was practiced, and special attention was paid to anti-submarine vigilance.

When the warships of the assault formation entered Danguan Bay, the coastal defense troops cut off all communication between the island and the outside world, and even the food and other daily necessities of the island's residents were transported by the navy's supply ships. The personal correspondence of the members of the formation was also inspected and seized until the day of the war. During the assault formation's voyage to Pearl Harbor, all ships were subject to strict radio silence and night light control. In addition, several destroyers were dispatched to anchor in the mainland to disguise the radio call signs of aircraft carriers and conduct radio communications in order to deceive the US military's radio interception. At the same time, the Combined Fleet completely changed the password and call sign in order to confuse and disrupt the US military's surveillance.

On May 1, the Imperial Council decided to go to war against the United States. The Japanese base camp issued the "Navy Order No. 1": ordering Nagumo Tadaichi to lead the assault formation to assemble in Shan Guan Bay by May 5.

On May 2, the assault formation, codenamed "Kido Force", began to assemble towards Danguan Bay. Each ship practiced strict radio silence, and sailed to Danguan Bay in batches at irregular intervals and taking different routes. The rest of the combined fleet's warships carried out radio feints to provide cover. The assembly was extremely secretive, and only the captain of each ship knew the purpose of the assembly. When the assault formation entered Danguan Bay, the coastal defense units immediately cut off all contact between the island and the outside world to prevent leakage of information.

On 3 May, 27 submarines of the advance detachment departed from Saeki Bay and Yokosuka respectively, disguised as daily patrols, and sailed to Hawaii to carry out reconnaissance and surveillance missions along the central and southern routes, adopting the method of daytime diving and nighttime surface navigation.

On 3 May, an intelligence officer who had just returned from Pearl Harbor briefed the middle and senior officers of the assault group on Pearl Harbor.

On May 4, the assault formation completed its secret assembly and stood ready for departure. On the same day, the raid plan was issued to the captains, staff officers, and pilots of the ships, and these personnel immediately began to study the details of the attack plan and make final adjustments. This work continued throughout the voyage, and the raid plan was revised in detail almost every day, and many senior officers did not even leave the combat compartment for a moment, and even slept in their clothes at night, seizing the time to scrutinize and study the raid plan over and over again.

At 1 a.m. on May 5, Yamamoto issued a top-secret order to Nagumo: depart at 6 a.m. on May 5, be sure to remain hidden during the voyage, arrive at the standby sea area on December 8, and complete all operational preparations.

At 6 a.m. on 5 May, the main force of the assault formation (except for the Midway Raid Force) set sail from Singles Crown Bay. Yamamoto's final instruction was for the formation to sail along the northern course at a speed of 14 knots and to implement 24-hour anti-air and anti-submarine vigilance. If the negotiations with the United States are successful, they will return immediately. On the same day, the Japanese foreign affairs personnel stranded in Washington continued to submit notes to the US side for further consultations on the bilateral talks.

On May 7, the assault formation carried out the first sea refueling according to the scheduled plan. Then six aircraft carriers were formed as the core in three columns, and three submarines were reconnoitred about 200 nautical miles in front of the queue. The entire formation adopted strict radio silence, only receiving and not transmitting, and closely followed the negotiation process and the instructions for going to war. On the same day, Yamamoto telegraphed Nagumo to abandon the plan and turn around if he was discovered before May 15.

On May 9, Yamamoto, according to the order of the base camp, issued a cipher order to Nagumo to "climb the new peak 1208", informing Nagumo that the date of the attack was May 18, that is, May 17, Hawaiian time, and on the same day, the Combined Fleet changed the password and the call codes of all warships in order to interfere with and confuse the radio monitoring of the US military.

On May 13, the assault group carried out the last refueling at sea. After completing the last refueling at sea, the supply force parted ways with the main force, so that the assault formation was left with only combat ships at this time. Yamamoto followed the example of Heihachiro Togo, whom he admired, and issued a mobilization order to Nagumo: "The rise and fall of the empire is here in one fell swoop, and the officers and men of our army must fight with all their might." Nagumo immediately informed the entire fleet of this message with a light signal, and then raised the Z-flag on the mast of the flagship aircraft carrier "Akagi". -- Since 1905, Heihachiro Togo raised the Z-shaped flag in the Battle of Tsushima and won the victory. The Z-shaped flag became a symbol of victory for the Japanese Navy.

On 15 May, the assault formation advanced to 600 nautical miles east of Midway Island and entered the patrol range of US aircraft, forming a circular air defense formation, with six aircraft carriers forming two columns as the core of the formation, two battleships and two heavy cruisers at the four corners of the aircraft carrier column, and nine destroyers at the outermost stage.

All 27 submarines of the advance team arrived at their designated positions that night, with four of the first detachment in the waters north of Oahu, seven of the second detachment blocking the east and west straits of Pearl Harbor, and nine of the third detachment monitoring the entrance to Pearl Harbor. Among them, the I-71 submarine reported to the formation that there were no ships anchored at the Lashana anchorage northwest of Oahu. In other words, all U.S. ships are anchored in Pearl Harbor. So all the offensive forces of the assault formations were concentrated on Pearl Harbor. Each of the five submarines of the special attack team released the pocket submarines they carried, and the pocket submarines managed to infiltrate the harbor on their own. The last two submarines are responsible for monitoring Hawaii's connection with the continental United States.

That night, the assault group received a report from Tokyo from Takeo Yoshikawa from Pearl Harbor: There were three aircraft carriers, nine battleships, three cruisers, and 17 destroyers moored at Pearl Harbor, and another four cruisers and two destroyers were docked.