The US has historically maintained a tiny peacetime army. If I remember correctly, in 1939 the US's army was comparable in size to Belgium's. And a large part of it was already in the Philippines.

For most of the 20s and 30s the US was doing war plans and they kept running into the same basic problems. The army was too small and treaties with Japan prevented the construction of forts and naval bases on pacific islands like Guam. So if war broke out, the military knew they weren't going to be able to hold the Philippines. And they wouldn't be able to easily sally and supply the fleet to relieve the troops in the Philippines.

So the Philippines were indefensible vs the Japanese. But the civilian government was not going to fund what would be required to actually make them defendable. Nor would anyone fully admit to that conclusion. So, everyone kept working on the problem with the hope that a solution would present itself.

And after the fall of France, it looked like one did, Congress opened the purse strings. They budgeted for massive military outlays. They started the peace time draft. And they funded the build up of the Philippine army. MacArthur even had a new defensive plan that looked like it would keep the Japanese from gaining a foothold in the islands.

Materiel was being rushed to the islands. Come spring of 1942 there would be a dozen trained and well equipped divisions, including an armor division, backed up by a large Air Force centered around a B-17 bomber force. Unfortunately the Japanese attacked six months earlier that MacArthur predicted. The men were there, but they weren't yet trained and they didn't have the equipment yet. So instead Mac only ended up with a much larger group than needed to suffer on Bataan.

Turns out the pre-WWII US way of building an army (that is only building it once the war has started or is just about to start) is a poor way to do it, when it requires a modern logistics train.

TLDR; Basically the army knew about it for decades. By the time they convinced congress to give them the budget to properly defend the islands it was too late... but perhaps only by months.

One reason that these Pacific holdings were unfortified was the Washington Naval Treaty. The main goal of this treaty was to put a limit on the size of the British, American, Japanese, French, and Italian navies. The Japanese were reluctant to accept this treaty as they relied on their navy to maintain their status as a Pacific power. In order to get Japan to accept the treaty, the US offered to not build any new fortifications in its Pacific holdings. This concession was vital in getting Japan to agree to the treaty and it worked.

Also, a few hours after Pearl Harbor was attacked, Manila was attacked as well. Gen. McArthur has been accused by some of indecision and/or being in conflict with then Philippine President Quezon to prevent hostilities there by attacking Japanese bases in Formosa.

When the Japanese did attack at midday Manila time, the planes that had been providing air cover at Clark Field outside Manila were on the ground refueling. Many of the planes at other local airstirips like Iba were almost completely wiped out.

The Philippines were hoping to remain neutral in the war, MacArthur and President Quezon were friends, and by striking back at the Japanese from Philippine bases, they would not be able to remain neutral. Once the bases were attacked directly, MacArthur had no choice. But upon realizing that most of his air cover had been destroyed, he ordered them to Australia to save what was left.

The surrounding islands with small garrisons fell quickly, Wake Island put up a helluva fight, and the southernmost large island was claimed within a month. They worked north from there, toward Japan.

The point about the treaties is a good one. The Japanese had started building up their bases in their mandates prior to the war in secret. The US considered doing likewise, but declined to until it was plainly obvious that war was coming.

There had been a very negative attitude towards any companies that did arms manufacturing post world war I, complete with unpleasant subpoena's in front of congress. While the US actually started building up prior to the war, it took a while for the gears to start moving. People remember the state of the army and navy at the end of the war, but it wasn't until late 1943 that supplies really started coming a way that changed the course of the war. As late as Guadalcanal, the US and Japanese supply chains and equipment where roughly comparable. There simply was not the capacity in many times to build up the forces to the level where they could have presented a significant challenge. Even Pearl Harbor didn't have enough aircraft to throw up a sustainable scouting perimeter.

The other huge factor was very poor military leadership at the start of the war. In particular, after Pearl Harbor, the Philippians was attacked, and a huge quantity of material - most notably aircraft was destroyed on the ground, despite the fact that Pearl had occurred earlier in the day. This helped open the door the fall of the Philippians.

Even if the US had the equipment in the field, the Japanese forces where war-tested and executing at a high (and unsustainable) level. Japan had been in a state of war for almost a decade, and were facing US and UK troops that where green. Despite the fact that the allies often had huge numeric advantages (more then 2:1 at Singapore for example), the Japanese forces were far more ready and committed to the battles.