Starting on 20 Dec, the Japanese Army landed on Mindanao and then Luzon, quickly capturing airfields and other key strategic positions.
At 1235 hours, Japanese Army fighters reached the airfield at Iba on the western coast of Luzon, destroying a flight of P-40 fighters in the process of landing.
A short time later, the Del Carmen airfield to the southeast was also attacked, with its outdated P-35A fighters forming little resistance against the more modern Japanese fighters.
The Filipino 1st Regular Division opposed the Lamon Bay landings fiercely and slowed the Japanese advance, but ultimately would not be able to hold the line.
While Japanese troops advanced across Luzon, President Manuel Quezon of the Philippines requested President Roosevelt to grant the Philippine Islands their independence so that he could announce Philippine neutrality.
Three airstrips at Luzon were taken very quickly, while the Lingayen Gulf region fell on 22 Dec.
Between 22 and 28 Dec, an additional 43,110 Japanese troops arrived via the beaches at Lingayen Gulf despite poor weather and rough seas.
In early Dec, Japanese bomber formations were observed flying within 20 miles of Lingayen Gulf beaches and returning to Taiwan, presumably making trial runs in preparation for the attack.
As a precaution, orders were given to move the 27th Bombardment Group B-17 bombers southward to Mindanao, out of range of the Japanese bombers.
At the start of WW2, the Philippine Islands were United States territory as per the 1898 Treaty of Paris.