At 8:44 p.m. (PST), February 11, 2010, a short-range threat-representative ballistic missile was launched from an at-sea mobile launch platform. Within seconds, the ALTB used onboard sensors to detect the boosting missile and used a low-energy laser to track the target. The ALTB then fired a second low-energy laser to measure and compensate for atmospheric disturbance. Finally, the ALTB fired its megawatt-class High Energy Laser, heating the boosting ballistic missile to critical structural failure. The entire engagement occurred within two minutes of the target missile launch, while its rocket motors were still thrusting.
However, the fact that they were able to do it from the ALTB during the boost phase of the missile launch (first two minutes!) is worthless. To do this during an actual ICBM attack would require /many/ ALTBs to be flying over enemy territory at the time of launch. This is because of simple physics. The earth is round. ICBM launches take place on the other side of the earth. The ICBM will no longer be in the super vulnerable boost phase when we can target them with the ALTB platform.
The immediate reaction is that satellites would solve this problem. But, there are two problems with that idea. First, to make a laser powerful enough to destroy/disable an ICBM requires a lot of space (there's a reason they use a 747 and it's not sex appeal). Satellites just aren't big enough or powerful enough or reliable enough to be effective platforms.... and we'd need /a lot/ of satellites. Second, there are treaties about weapons in space that no one wants to break. Modern society (particularly the United States) are too heavily invested in delicate satellites for communications, commerce, etc. Once weapons are allowed up there... back to the industrial era we go.