The United States is in the midst of an ambitious effort to build and deploy a wide range of ballistic missile defense systems. Proponents of the systems argue that they will be effective against a host of current and postulated threats from ballistic missiles. In reality, success or failure will depend not only on the technology used in defenses, but also on the tactics and technologies used in missile attacks. Resourceful and determined attackers seek to stress a defense beyond its limits. They can do this by altering their weapons' characteristics with tactics or devices known as countermeasures, such as maneuvers, decoys, and infrared and radar stealth, which hinder or prevent the defense from identifying or hitting their incoming missiles. The ability of a defense to adapt to and deal with countermeasures is the ultimate test of its combat worthiness.