The Japanese plan was based on a detailed knowledge of the Philippine Islands and a fairly accurate estimate of American and Philippine forces. The Japanese were aware that the bulk of the American and Philippine forces was on Luzon and that the U.S. Army garrison had been increased since July 1941 from 12,000 to 22,000. Eighty percent of the officers and 40 percent of the enlisted men were thought to be Americans and the rest, Filipinos. American troops were regarded as good soldiers, but inclined to deteriorate physically and mentally in a tropical climate. The Filipino, though inured to the tropics, had little endurance or sense of responsibility, the Japanese believed, and was markedly inferior to the American as a soldier. The American garrison was correctly supposed to be organized into one division, an air unit, and a "fortress unit" (Harbor Defenses of Manila and Subic Bays). The division was mistakenly thought to consist of two infantry brigades, a field artillery brigade, and supporting services. The Japanese knew that Mac Arthur also had one battalion of fifty-four tanks-which was true at that time-and believed that there was also an antitank battalion in the Islands. The harbor defenses were known to consist of four coast artillery regiments, including one antiaircraft regiment.

The Japanese estimated that the American air force in the Philippines was composed of one pursuit regiment of 108 planes, one bombardment regiment of about 38 planes, one pursuit squadron of 27 planes, and two reconnaissance squadrons of 13 planes. American aircraft were based on two major fields on Luzon, the Japanese believed. They placed the pursuit group at Nichols Field, in the suburbs of Manila, and the bombers at Clark Field. Other fields on Luzon were thought to base a total of 20 planes. The Japanese placed 52 Navy patrol and carrier-based fighter planes at Cavite and 18 PBY's at Olongapo.

The strength of the Philippine Army and the Constabulary, the Japanese estimated, was 110,000 men. This strength, they thought, would be increased to 125,000 by December. The bulk of the Philippine Army, organized into ten divisions, was known to consist mostly of infantry with only a few engineer and artillery units. This army was considered very much inferior to the U.S. Regular Army in equipment, training, and fighting qualities.

Though they had a good picture of the defending force, Japanese knowledge of American defense plans was faulty. They expected that the Philippine garrison would make its last stand around Manila and when defeated there would scatter and be easily mopped up. No preparation was made for an American withdrawal to the Bataan peninsula. In October, at a meeting of the 14th Army staff officers in Tokyo, Homma's chief of staff, Lt. Gen. Masami Maeda, had raised the possibility of a withdrawal to Bataan. Despite his protests, the subject was quickly dropped. Staff officers of the 48th Division also claimed to have discussed the question of Bataan before the division embarked at Formosa. The consensus then was that while resistance could be expected before Manila and on Corregidor, Bataan "being a simple, outlying position, would fall quickly."