"Is Homeland Security really so good that it has prevented a whole slew of attacks?"
Maybe it's worth the billions being spent on it.

"In Israel, where I suspect security is far higher than in the US, attacks happen regularly. If there was the same burning desire to attack in the US, I can't imagine that all would be prevented. I mean, a sniper on the roof firing into the crowds coming out of the blacked-out NY subways, how hard would that have been? Didn't happen. A gasoline bomb in a rock concert, a shrapnel suicide bomber in a suburban mall, a fertilizer and diesel fuel truck bomb at a high school... they didn't happen.

Why not?"
A bit wistful?? Maybe because we are at war with terrorists. The Japanese didn't really effectively attack the West Coast after Pearl Harbor. And lost the battle when they tried to attack the tiny island of midway.
Why not?

"Is Bush's anti-terror campaign that effective?"
The thought is kinda of scary isn't it?

"I think that only an omnipotent god squarely on your side could have prevented such things from happening IF THERE WAS A PERVASIVE AND SERIOUS THREAT."

Oh my, a lefty believing in God, now things are really getting really bad.

"Since they didn't happen, I'm left to conclude that such a threat doesn't exist. Not that there are no real terrorists anywhere, of course, but that there are none who are really serious about wreaking terror on US soil."
Good idea. Stick your head in a hole.

"And that has a lot of implications: justification for war on Iraq, the many questions about 9/11, the gullibility of a large number of Americans, the motivations and morals of certain government officials..."

Yes, lots of implications. Like for instance, Bush winning the second term and picking up seats in Congress.

"Does America need enemies so badly that it has to invent them?"
Like for instance, the Lefties saying Bush is worse than Hilter- apparantly some Americans need to invent enemies.

"And what really drove the 9/11 hijackers? "
A few turbine jet engines.