A report published by the libertarian think tank the Cato Institute warns that the fruits of technological innovation could mean a new range of unexpected threats to the United States. Terrorists and small states equipped with weapons that take advantage of new technologies could devastate U.S. forces abroad--and even strike the homeland. Hammes, a retired U.S. Marine Corps infantry officer and expert on so-called "asymmetrical warfare"--warfare in which combatants avoid enemy strengths to strike weaknesses--warns that technologies such as 3D printing, nanotechnology, space and near-space travel, drones, and artificial intelligence could lead to cheap, inexpensive weapons with intercontinental reach, devastating lethality, and the ability to overwhelm U.S. defenses. According to Hammes, the linchpin of these capabilities is additive manufacturing, or 3D printing. Combine those two and you have a tiny tank hunter that can be difficult for tank crews to shoot down.