The defense industry has become broader since the beginning of the 21st century. Prior to that time, the defense industry was almost entirely composed of companies that specialized in the research, development, and production of defense weaponry and technologies. After the terrorist attacks of September 11, 2001, the Department of Homeland Security became an important part of national defense, and many new technologies and products were developed to meet the department's increased needs and the needs of more stringent security at airports and other public places.

The primary goal of the defense industry is to develop and manufacture products that the U.S. military and Department of Defense can use in the protection of the country. As some products are developed, other more consumer-friendly uses can also lead to more widespread use. For the most part, however, defense products are typically very expensive, and manufacturers contract with military branches or departments to produce them. Humankind has been fashioning weapons for self-defense for centuries. Some of the earliest examples include the Greeks' use of incendiary weapons as early as the fourth century B.C. Gunpowder, invented by the Chinese, became one of the most important developments in the art of war and became widely used everywhere in the world by the 18th and 19th centuries. By the 14th century, armies had adapted gunpowder for use with cannons, which could fire large stones or metal balls for long distances and destroy a city's fortifications. Mortars, developed by the Dutch in the 17th century, and other explosive bombs were used to terrible effect. The defense industry as it is known today first developed during World War I and World War II with the use of airplanes, jeeps, and other vehicles in battle. Manufacturers that produced similar goods turned to the manufacture of war goods, and many continued to produce them after the war was over. Even industries that previously had little to ...