A terrorist group has succeeded in infiltrating 45 highly-trained members into the U.S. They comprise five 9-man teams each located in a different part of the country. Each team has military assault rifles, handguns, grenades, explosive charges and detonators. The terrorist group overseeing the operation has carefully chosen the targets for each team. Each target is in either a small city or a large town, well away from possible military reaction forces or a major police presence. Number 1's target is a college campus, #2 a popular hotel/casino, #3 a large mall, #4 A 200 child daycare facility, and #5 a senior care complex. Each terrorist team will time their strikes to occur simultaneously. During each attack the teams subdivide into three 3-man groups to hit different areas at once and spread maximum panic. For example, team #1 section A will attack the campus police HQ, to kill the police or at least disrupt their response. Section B will attack a crowded auditorium, while Section C invades the freshmen dorms. Each team divides targets in similar fashion. Attacks are supplemented with the detonation of concealed explosive charges that were place immediately prior to the actual attack, if possible. For example, Team #3 could carry them into the mall concealed in shopping bags and simply drop them into a couple trashcans, to be detonated remotely once they start their attack.
Is there any way to prevent such an attack? Yes, if you discover it before it can be initiated. If not, no military response teams, or militarized police units are going to be able to stop it. The best they can do is try to contain the carnage. Barring the lucky presence of police officers in the right place at the right time, armed civilians are about the only chance of stopping an attack once it gets initiated. Consider target #4, the daycare. As with the other teams the terrorists split into three sections. Section A enters the front of the building, Section B the rear, and Section C the outside play area. This is probably the absolute worst target to defend, since most of the occupants are completely helpless small children. But with this type of target, the terrorists will expect no serious resistance. What if one of the teachers held a concealed carry permit and had her weapon on her? Three terrorists burst into the building through the front door shooting, killing children and staff alike. The teacher is not hit in the initial fire, draws her gun and returns fire, killing one terrorist and wounding another. Many of the children and staff are still going to die, but this changes the entire equation. The remaining member of the terrorist section and his wounded comrade have to focus on this threat instead of continuing to kill the helpless. In the confusion and temporary delay of the slaughter, maybe some of the children could flee to safety. Unlikely, yes, but at least there's a small chance. And this is the worst of all five target scenarios.
In a large public area such as a mall, there could conceivably be a few dozen or more armed civilians capable of reacting to an attack. Some of them might even be ex-military or law enforcement. They pose a serious potential threat to this type of terrorist strike, and one that cannot be fully accounted for in terrorist planning. Chances are they won't be able to stop such an attack cold, but they could disrupt it or manage to stop it short of the number of casualties that would otherwise be inflicted. Even one armed civilian who is willing and able to act is a danger to the terrorists. Unarmed civilians on the other hand are basically helpless, and have few good options. When help arrives it will almost certainly be too late for many of them. In my opinion, small-scale terrorist attacks directed at civilians are best countered by the civilians themselves.
Note -- Pure suicide bomber type attacks are even harder to defend against. Unfortunately, they are often identified after they explode, and then its too late.