Such systems, with varying degrees of capability, are currently in use by at least thirty different states. The US Navy, for example, has used the Phalanx gun and Aegis missile defense systems to defend its ships for decades. Though far less capable than the intelligent machines of today and tomorrow, these systems can be switched into a fully automatic mode that enables them to close the kill chain against incoming missiles without human involvement. The decision to trust those machines to do so was born of necessity: it was unlikely humans could respond fast enough to counter incoming missiles. That inability was deemed a greater danger than the option of turning the kill chain over to a machine that could shoot down missiles in time-sensitive situations more effectively than humans could.