Chapter 1 introduces tactical air power (TAP) theory to explain why, how and when modern air power works. After World War II, two technologies changed the character of air warfare. In the Cold War the proliferation of thermonuclear weapons and the exorbitant costs anticipated from nuclear war deterred the United States and the Soviet Union. The second technology was the proliferation of radar- and infra-guided air-to-air and surface-to-air missiles, which increased the lethality of the air domain and with it the United States shifted from the bomber to tactical aircraft (tacair) as its primary combat platform. This book examines modern US air power in the Vietnam War. In Vietnam and the modern air wars that followed, US air power has been more effective in directly attacking enemy fielded forces, rather than by independent strategic bombing and air interdiction. Joint air–ground operations place the enemy on the horns of a dilemma: to mass and maneuver only to be susceptible to air attack or disperse and hide and be vulnerable to an opposed army’s attack. With the right combination of air superiority, air-to-ground capability, and a capable ground force, under the right environmental conditions, air power can disrupt an enemy’s strategy.