van der Waals forces: see intermolecular forces.

intermolecular forces, forces that are exerted by molecules on each other and that, in general, affect the macroscopic properties of the material of which the molecules are a part. Such forces may be either attractive or repulsive in nature. They are conveniently divided into two classes: short-range forces, which operate when the centers of the molecules are separated by 3 angstroms or less, and long-range forces, which operate at greater distances. Generally, if molecules do not tend to interact chemically, the short-range forces between them are repulsive. These forces arise from interactions of the electrons associated with the molecules and are also known as exchange forces. Molecules that interact chemically have attractive exchange forces; these are also known as valence forces. Mechanical rigidity of molecules and effects such as limited compressibility of matter arise from repulsive exchange forces. Long-range forces, or van der Waals forces as they are also called, are attractive and account for a wide range of physical phenomena, such as friction, surface tension, adhesion and cohesion of liquids and solids, viscosity, and the discrepancies between the actual behavior of gases and that predicted by the ideal gas law. Van der Waals forces arise in a number of ways, one being the tendency of electrically polarized molecules to become aligned. Quantum theory indicates also that in some cases the electrostatic fields associated with electrons in neighboring molecules constrain the electrons to move more or less in phase.

air forces, those portions of a nation's military organization employing heavier-than-air aircraft for reconnaissance, support of ground troops, aerial combat, and bombing of enemy lines of communication and targets of industrial and military importance.

Early Military Use of Aircraft

The history of air forces begins with the use of balloons by French forces in Italy in 1859 and by Union forces in the U.S. Civil War. Balloons thereafter proved useful as a means of observation, but air forces in the modern sense date from World War I, when the offensive capabilities of the airplane were first demonstrated. The somewhat tentative use of scout planes at the beginning of the war was followed by the creation of small forces of fighter planes that engaged in aerial combat and bombing raids. Although Germany took the lead in air strategy, the Allies soon closed the gap. Indeed, throughout World War I, such development and counterdevelopment accounted for the rapid advance of military aeronautics. The use of aircraft for reconnaissance, which made control of the skies important to military operations, resulted in the development of aerial combat, which led to formation flying, dogfights, and the bombing of enemy lines of communication and munitions depots.

Evolution of the Modern Air Force

As the effectiveness of aircraft as a tactical weapon increased, consideration was given to the establishment of air forces independent of a nation's ground forces. After the war a few allied strategists, including Giulio Douhet and others, such as Gen. William Mitchell of the United States, fought for the intensive development of airpower and pleaded for large air forces, arguing that future wars would be won by strategic bombardment of an enemy's industrial centers, thereby destroying the economic means of conducting a war. In the 1920s and 1930s the French, British, and Italians used airplanes for reconnaissance and strategic bombing in colonial wars in Africa, the Middle East, and India. These experiences, combined with the rapid and extensive advances in aeronautical technique that followed World War I, resulted in a much broader application of airpower in World War II.

During World War II

During the 1930s, Germany devoted great efforts to air armament and the early days of World War II seemed to uphold Hitler's boasts of the effectiveness of the Luftwaffe (air force) under Hermann Goering. This was especially true of tactical air support for the ground troops, which was a crucial part of Germany's successful form of mechanized warfare, the blitzkrieg. The first great air battle in history was the Battle of Britain, in which the British Royal Air Force defeated the German Luftwaffe (1940) over Britain. In the Pacific, Japan entered the war with a stunning air attack launched from aircraft carriers on Pearl Harbor.

The subsequent development of airpower greatly altered the nature of warfare, and the use of aircraft over both land and sea played a major role in nearly all of the important engagements of World War II. Airplanes were used for strategic and tactical bombing, attacking of naval and merchant ships, transportation of personnel and cargo, mining of harbors and shipping lanes, antisubmarine patrols, photographic reconnaissance, and support of ground, naval, and amphibious operations. Throughout the war, the British and U.S. air forces conducted massive strategic bombing of Germany, but postwar bombing surveys showed it was not decisive in the Allied victory. In the Pacific, U.S. carrier-based aircraft by the end of 1944 had destroyed the Japanese fleet and air force. In the last months of the war, Japan itself was subjected to intense strategic bombardment, ending with the dropping of atomic bombs on Hiroshima and Nagasaki. Other major developments of World War II included improved techniques of flying and aircraft design and an accumulation of geographical and technological knowledge essential to modern aviation. By the end of the war, the importance of airpower was accepted by all.

Postwar Use of Airpower

Since World War II, the increased role of helicopters has been a major development, allowing for increased air support of ground troops. In the Korean War air forces of the United Nations Command effectively enveloped the North Korean army and later cut supply arteries to Chinese Communist troops so that an armistice could be negotiated. Similar ground-air tactics were employed by the United States in Vietnam, while the North Vietnamese made effective use of Soviet-built ground-to-air missiles and tactical air support. The Persian Gulf War, which saw the introduction of stealth fighter planes (see stealth technology), was the first unambiguously decisive airpower victory in warfare, but even there the conflict was only ended after the ground forces attacked. Airpower was also used fairly effectively, although with less than immediate results, by the North Atlantic Treaty Organization to force the capitulation of the Yugoslavia during the Kosovo crisis in 1999. Fighting in Afghanistan (2001) saw precision-guided smart weapons become the predominant ordnance, but these were often targeted most effectively when the air forces worked in conjunction with spotters on the ground.

The development of nuclear weapons, jet propulsion, the guided missile, and satellites has widened the concept of airpower and the role of air forces. The U.S. Air Force (see Air Force, United States Department of the) now refers to aerospace power (instead of airpower) and considers space a crucial military theater. Air forces also have come to assume a primary strategic role in deterring war by employing in readiness a second-strike retaliatory force (see nuclear strategy) consisting of both aircraft and missiles. In the United States this mission was carried by the Strategic Air Command, which has been replaced by the interservice Strategic Command.

Relatively weak electrical forces that attract neutral (uncharged) molecules to each other in gases, liquefied and solidified gases, and almost all organic liquids and solids. Solids held together by van der Waals forces typically have lower melting points and are softer than those held together by ionic, covalent, or metallic bonds (see bonding). The forces arise because neutral molecules, though uncharged, are usually electric dipoles, which have a tendency to align with each other and to induce further polarization in neighbouring molecules, resulting in a net attractive force. They are somewhat weaker than the forces involved in hydrogen bonding. Seealso Johannes D. van der Waals.

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Military organization of citizens with limited military training who are available for emergency service, usually for local defense. In many countries the militia is of ancient origin. The Anglo-Saxons required every able-bodied free male to serve. In colonial America it was the only defense against hostile Indians when regular British forces were not available. In the American Revolution the militia, called the Minutemen, provided the bulk of the American forces. Militias played a similar role in the War of 1812 and the American Civil War. State-controlled volunteer militias in the U.S. became the National Guard. British militia units, begun in the 16th century for home defense and answerable to the county sheriff or lord lieutenant, were absorbed into the regular army in the 20th century. Today various paramilitary organizations, from U.S. white supremacists to revolutionaries in the developing world, use the term militia to accentuate their populist origins.

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