The issue here is, as has been stated by many others, is that there are simply too many areas of the US that are vulnerable to this type of activity. A determined person can obtain their goals no matter what so long as we have a free society. The only way to stop the terrorists from inflicting any harm on the US is an absolute lockdown of the US. By that I mean that no one for any reason is permitted to go anywhere or do anything. However, this is just not the case, and would not be acceptable to people today.

As for creating a threat, it all depends on what the person(s) want to do. Do they want the shock and awe of a single large scale attack, or do they want a series of smaller less "damaging" attacks that cause the populace to panic and change the way they live. A large scale attack is harder to execute, but easier for complete sucess from the terrorists point of view. To achieve this goal all that would be needed is a number of teams attempting to create the lage scale attack. 9-11 was this approach. From what I have read OBL had assumed that maybe only one or two of the teams would have suceeded. He figured the others would be stopped in one form or another. Either our security, the passengers, the military, or the teams one of those would fail the "mission". The fact that 3 of the teams were sucessfull was what made the attack even more sucessful in his eyes.

In contrast a number of small event attacks are significantly harder to pull off, but the fear placed by these is where they have the biggest sucess. The two examples I could see is the suicide bomber, and the left item. Imagine if you will, if all of the sudden we started having bombers in our malls, restraunts, gas stations etc. People would stop going to these places. All one has to look at is the DC Sniper case to see how this can paralyze a city. If this were taken to a national level, in several cities both large and small, the effect on the populace would be greater. Given our hysteria and our media, I would venture that it would only take 10 or 15 of these events to effectively curtail our retail enviornments. The security reactions would be enough to deter many. The real point of terrorism is to make people feel unsafe in the areas where they are used to feeling safe.

Once this objective had been met, the terrorists would just move to our next "soft target". For example, what would stop the terrorists from filling a few dozen trucks with fertilizer and blowing them up on bridges that cross the Mississippi. OK City showed what those can do. 3 of these style attacks would bring trucking and transportation to a halt. Then the terrorists move to the next target. The cycle repeats.

In the end we can come up with all these senarios, but we cannot stop them. You cannot stop the enemy who does not play by the rules, so to speak. All we can do is hope to educate the people that we cannot be 100% safe 100% of the time. Everything we do to make something safe creates a weakness elsewhere, that can be exploited. Once people learn to accept that this can happen no matter what we do, then we have won the war. However, until we do we are fighting a losing battle.