10 U.S.C. § 5501

1. (1)to provide for the common defense of the United States and its citizens by deploying and maintaining a next-generation missile defense shield;
2. (2)to deter and defend the United States, citizens of the United States, and critical infrastructure of the United States, against the threat of foreign attack by increasingly complex ballistic, hypersonic glide, and cruise missiles, and other advanced aerial threats; and
3. (3)to guarantee the viability of an effective nuclear response capability of the United States and to support the continued deterrence of strategic attacks against the homeland of the United States.