The T3 strategic missile defense, SMD for short, are stationary structures launching ordinance to defeat incoming ICBMs directed towards the protected area.
SMDs are built in the same fashion as the SMLs, with both launcher and ammunition to be built. Once loaded, Defense Missiles are launched automatically as soon as a nuke enters range, however it still must be reloaded after use. Nukes, launched by the player or allies, will not be targeted.
Counter-nukes are visible on the map, abeit no audio cue, shown as a red no-entry symbol; notifiing the attacking commander that the target has active anti-nukes in the area.
It is a valuable defense if you're expecting heavy use of nuclear armaments and because SMDs are built and loaded faster than SMLs, alert ACUs may often so-called nuke-proof their bases. Furthermore, base size should be considered with building more than one SMDs, so that their range can cover the expanse of the facility; Preventing a shrewd commander from landing a precise strike on outlaying buildings.
Anti-Nukes build faster than nukes, for far less resource investment and a single launcher can protect against multiple simultaneous nuclear threats. In-game multiplayer tests confirm that a single fully loaded launcher is able to shoot down up to 5 simultaneous nukes.
- Forged Alliance
As far as factional differences go, the only notable one is the Aeon SMD. It is by far the best Strategic Missile Defense. It alone can shoot down up to 5 missiles at a time, as can be seen here, in the link below:
It is also worth noting that all five missiles in the video were launched directly at the SMD. If the final defensive missile had to travel to the strategic missile because it wasn't aimed at the SMD, the final nuke would likely have landed before interception was possible, given in the referenced video the SMD kills it with mere nanoseconds remaining. The extra time taken for the SMD missile to reach the missile when it is not coming directly towards the SMD would cause this. This is however, unconfirmed.
One SMD can shoot down two nukes in the time it takes them to go from descending to impact in non-FA Supreme Commander. Thus you will need (number of enemy SMDs * 2) + 1 simultaneously launched missiles (this is the key part - three missiles in a chain won't break through one SMD's defensive perimeter unless it lacks missiles!) to overwhelm enemy SMDs. Aeon SMDs can sometimes get lucky and shoot down three missiles, but this is generally rather rare. It'll probably be more effective to simply overwhelm the enemy with the troops you could've bought with the resources spent on the missiles, though it is a good way of proving a point when you have the resources to spare.
The use of multiple SMD's in multiplayer is due to the fact that the enemy will have to take out multiple of these vulnerable buildings, which when spaced apart makes this no simple job. Additionally, as discussed above, it is possible a SMD may be overwhelmed, whereas multiple SMD's can fire simultaneously at multiple strategic missiles. The buildpower of SMD's is very high in Forged Alliance, which means when expecting multiple nukes soon, it's best to have multiple SMD so they can all build missiles. The benefits of this are often outweighed by the cost of building the actual SMD building's themselves in the early endgame.
For more information on strategic warfare, see T3 strategic missile launcher