Luzon (; Tagalog pronunciation: [luson]) is the largest and most populous island in the Philippines. Located in the northern portion of the Philippines archipelago, it is the economic and political center of the nation, being home to the country's capital city, Manila, as well as Quezon City, the country's most populous city. With a population of 53 million as of 2015, it contains 52.5% of the country's total population and is the fourth most populous island in the world. It is the 15th largest island in the world by land area.

Luzon may also refer to one of the three primary island groups in the country. In this usage, it includes the Luzon mainland, the Batanes and Babuyan groups of islands to the north, Polillo Islands to the east, and the outlying islands of Catanduanes, Marinduque and Mindoro, among others, to the south. The islands of Masbate, Palawan and Romblon are also included, although these three are sometimes grouped with another of the island groups, the Visayas.

The Philippines campaign (Filipino: Kampanya sa Pilipinas or Labanan sa Pilipinas, Spanish: Campaña en las Filipinas del Ejercito Japonés, Japanese: フィリピンの戦い, romanized: Firipin no Tatakai), also known as the Battle of the Philippines or the Fall of the Philippines, was from December 8, 1941, to May 8, 1942, the invasion of the Philippines by the Empire of Japan and the defense of the islands by United States and the Philippine Armies during World War II.

The Japanese launched the invasion by sea from Formosa, over 200 miles (320 km) north of the Philippines. The defending forces outnumbered the Japanese 3-2 but were a mixed force of non-combat experienced regular, national guard, constabulary and newly created Commonwealth units. The Japanese used first-line troops at the outset of the campaign, and by concentrating their forces, they swiftly overran most of Luzon during the first month.

The Japanese high command, believing that they had won the campaign, made a strategic decision to advance by a month their timetable of operations in Borneo and Indonesia and to withdraw their best division and the bulk of their airpower in early January 1942. That, coupled with the defenders' decision to withdraw into a defensive holding position in the Bataan Peninsula and also the defeat of three Japanese battalions at the "Battle of the Points" and "Battle of the Pockets", enabled the Americans and Filipinos to hold out for four more months. After the Japanese failure to penetrate the Bataan defensive perimeter in February the Japanese conducted a 40-day siege. The crucial large natural harbor and port facilities of Manila Bay were denied to the Japanese until May 1942. While the Dutch East Indies operations were unaffected this heavily hindered the Japanese offensive operations in New Guinea and the Solomon Islands, buying time for the U.S. Navy to make plans to engage the Japanese at Guadalcanal instead of much further east.Japan's conquest of the Philippines is often considered the worst military defeat in US history. About 23,000 American military personnel, and about 100,000 Filipino soldiers were killed or captured.