When people talk about US ABM systems they typically refer to the
Safeguard System that was limited (effectively banned) by the ABM
Treaty made with the Soviet Union. The Safeguard System was actually
developed and partially deployed. It was based upon a highly
sophisticated nuclear hardened phased array radar, a long range
nuclear tipped anti-ballistic missile called "Spartan" and an
incredibly fast accelerating short range anti-ballistic missile
called "Sprint". One or both(?) missiles used special thermonuclear
warheads that produced high radiation with low fallout (an ancestor
to the "neutron bomb").

Both the United States and the Soviet Union realized that ABM systems
were "destabilizing" when both sides were armed for Mutual Assured
Destruction (MAD). The Safeguard ABM system was a highly effective
design for stopping a limited nuclear strike against the US. However
the problem with a system like Safeguard was its effectiveness depended
upon its radar. If the radar was blind then the system could not
target the ABMs. The Spartan missile was an exo-atmospheric ABM. When
it's high radiation warhead detonated above the atmosphere, it would
ionize the upper atmosphere making it opaque to radar. Also any
satellite that was above the horizon and not super radiation hard would
also be destroyed by the ABM's explosion. Thus using the Spartan missile
would make the US temporarily blind to a second wave of Soviet missiles.
This is why Sprint was developed. Sprint was an endo-atmospheric
ABM. Sprint could destroy Soviet reentry vehicles (RVs) when they flew
beneath the plasma layer in the upper atmosphere created by Spartan.
Unfortunately Sprint also ionized the atmosphere. If the Soviets
attacked the United States with multiple waves of ICBMs, only the
first two waves would be destroyed. The Soviet's could easily invoke
a multiple wave attack since they had over 10,000 ICBM RVs (insane!!)
for MAD. To make matters worse it became clear that an ABM system
like Safeguard could only be effective if it was used in conjunction
with a first strike. For example: the Soviets could launch a multiple
wave first strike against the United States. The United States could
either wait and absorb the first strike hoping that our remaining
missiles would survive for a counter attack (not likely!) or launch
everything on warning in a single wave and hope they survived the
Soviet equivalent of our Safeguard system (equally unlikely). This
scenario made it clear that whoever developed a Safeguard ABM system
first would immediately use it or face nuclear annihilation.

Both the United States and Soviet Union saw this was the path
towards mutual suicide so both nations agreed to the ABM treaty.

Anyway it was because of the experience with Safeguard along with
the ABM treaty that the United States opted for an optical non-nuclear
ABM system when the ABM concept was revisited under Star Wars.
IMHO attempting to build an ABM system while the Soviet Union still
had MAD capability was a phenomenally stupid decision (one of
many under the Reagan administration). The Soviets could only have
interpreted the Star Wars exercise as an attempt to develop first
strike capability. Of course, Reagan's defenders like to say that
was the whole idea. They argue that Star Wars was a bluff intended
to scare the Soviets into redirecting the last meager parts of their
economy towards strategic weapons thus insuring their economic collapse.
However I don't think the US was bluffing with Star Wars. I think Ed
Teller had convinced Reagan that it would work (the Excaliber X-ray
laser ABM concept) and that a first strike tactic was feasible.
Fortunately Teller was wrong (Excaliber didn't work) and the Soviet
Union did implode. We (the human race) were incredibly lucky.

So what should we do with NMD? I don't think NMD could possibly be
better than Safeguard for defending the US against a limited attack.
Bush signaled that same belief when he indicated willingness to terminate
the ABM treaty. However no one seems to be talking about bringing back
Sprint and Spartan (a known workable system). They're still talking
about kinetic energy kill vehicles with optical seekers. IMHO ABMs are
a waste of time and money. The only reason why I see people still
pursuing this concept is because the military industrial complex wants
a little more suck time on the government tit. IMHO, Libya, Iran,
North Korea, etc. would most likely attack us with nuclear terrorists.
The nuclear weapon would be assembled near the target and not delivered
with a ballistic missile. Our best defense is a strong believable offense.
Our current tactic of obsolete offensive weapons based upon mutual assured
destruction with the Soviet Union is not credible. IMHO, we need to reduce
our nuclear weapons inventory to less than 1000 submarine launched ballistic
missiles each with a single advanced maneuverable reentry vehicle. The
warheads in these missiles should be based upon low radiation, low fallout
nuclear explosives. We want to be in the situation that we could subject
Iran/Libya to pinpoint nuclear economic castration but not slaughter the
friendly populations of Egypt, Saudi Arabia and/or Kuwait in the process due
to nuclear fallout.

Gary