US satellites would provide unambiguous warning of any threatening missile developments. If any country were developing a long-range missile, with or without foreign assistance, the United States would have considerable warning. Unlike nuclear weapons, developing missiles requires flight testing in the atmosphere, which US satellites would detect. A transfer of long-range missiles is extremely unlikely. Long-range missiles are very large and therefore extremely difficult to transfer undetected or to steal without the knowledge of the state authorities.
Ballistic missiles are an unlikely method of attack against the United States.
If a developing country did seek to attack the United States with weapons of mass destruction, ballistic missiles are probably the least likely method it would choose. First, US satellites would pinpoint the origin of any missile attack, allowing quick retaliation. Such retaliation will always be a powerful deterrent to missile attacks. Countermeasures would render national missile defenses ineffective. If the United States deploys defenses, it must expect that any long-range missile deployed by a hostile country would be equipped with countermeasures, since these would be far simpler to build than the missile itself.