Luzon, with one tenth of its total area a large plain, and another 5,000 square miles forming a magnificent river valley, is the one island in the Philippines whose terrain permits military operations on a large scale. In the north, closed in by mountains on the east and west, is a valley extending southward for over 120 miles and with an average width of 40 miles. Flowing north through the valley is the Cagayan River. Along the west coast is a narrow plain offering only limited opportunities for military operations. A road runs along this coast from the tip of Luzon towards Manila. Southern Luzon is a volcanic region, part plain and part mountain, with numerous deep indentations forming bays and gulfs, and with many beaches suitable for the landing of a military force.

The central portion of Luzon is composed of a plain extending north about 120 miles from Manila Bay to the Lingayen Gulf. With mountains to the east and west, the plain is well protected from invasion except at its two extremities. It is the most highly developed economic area in the Philippines and the one most suitable for mobile warfare.