From Japan Runs Wild, 1942–1943, by Peter Harmsen (War in the Far East, Book 2; Casemate, 2020), Kindle p. 15:
The Japanese pilots flying over the Philippines in the days after Pearl Harbor were experiencing an entirely new form of war. Many of them were veterans from the long conflict in China, where they had been facing Chinese and sometimes Soviet aviators in Russian-built planes. Now, in the midst of the crucial battle for control of the Philippines skies as a prelude to the planned invasion of the islands, they were up against Americans in US-built aircraft. They found to their relief that their own Mitsubishi A6M “Zero” fighter was a more than adequate match for the American P-40, as the Japanese plane outshone its US counterpart in everything except diving acceleration. “The confidence of our fighter pilots continued to grow, nurtured by the absence of effective opposition,” wrote Shimada Kōichi, on the staff of the Eleventh Air Fleet, which had the responsibility for air operations over the Philippines.
The Philippines, under the overall command of General Douglas MacArthur, was not Wake. And yet, despite its much larger size and more awe-inspiring defensive potential, it was essentially just another piece of US-held territory that the military planners in Washington had to effectively abandon beforehand. In the tense years prior to the outbreak of war in the Pacific, the US government, faced with the likelihood of being sucked into the ongoing war in Europe, had been forced to allocate desperately scarce military resources elsewhere. “Adequate reinforcements for the Philippines at this time,” according to General George C. Marshall, the US Army’s chief of staff, “would have left the United States in a position of great peril should there be a break in the defense of Great Britain.” What was not clear at the time was the fact that the Japanese were similarly constrained, and that the Japanese high command intended to take the Philippines with the smallest feasible number of troops.