Strategic missile launchers in Supreme Commander are nuclear weapons. They are considered strategic because they can target any part of the map. The explosion from a strategic missile is large and does tremendous damage, but it does not necessarily destroy every unit in its area of effect.
As with tactical missile launchers, the launch site must be built first, and then each missile must be built. The only effective countermeasure is a strategic missile defense unit. In a protracted conflict that involves strategic missile launchers countered by strategic missile defense units, it is possible to overwhelm the defenses. To do this, build several strategic missile launchers and fire them simultaneously at the same target. The strategic missile defense unit(s) must go through a firing cycle and use up their ammunition, so they may not be able to stop all of the incoming missiles.
When a player launches a nuclear missile, other players will hear the warning "Strategic launch detected". The icon for this missiles is a yellow icon resembling the nuclear hazard symbol, so opponents can track the movement of the missile and possibly move some faster units out of the anticipated target area.
Unlike real nuclear weapons, the concept of mutual assured destruction does not always to Supreme Commander.