It's hard to say, I think that its possible that Al-Qaeda was used by the CIA/Mossad to bring down the towers but this is really just speculation. That's probably where the bodies are buried if there was a conspiracy; not sure if I believe all the talk about pre-set bombs in the buildings and thermite set in advance by some nefarious entity. We probably won't know for sure until The Kingdom collapses and declassified KSA files start pouring out. The classified pages of the 9/11 commission suggested that the plot was launched with possible complicity of high-ranking people in Saudi Arabia but doesn't prove that the KSA directed it.
Assuming there was no conspiracy what remains to be explained by historians is how Al-Qaeda went from attacking the US in 2001 to acting as their puppets (again) in Libya and Syria.
Usually, these type of discussions lead to talk about "conspiracies" in the abstract and whether they are true or not. I think its necessary to split absurd conspiracies from plausible ones and objectively evaluate the evidence for the latter. Even people who claim to be materialists are not always the best at evaluating evidence. I'll be honest, that researching the JFK assassination has sort of turned me off from believing in your typical run-of-the-mill burgerland conspiracies.
While the notion that factions in the US government and its ruling class wanted JFK killed is plausible, I think when you look at the evidence for the assassination the theories put forward so far by skeptics (including leftist Peter Dale Scott) do not pass the test–at least it isn't enough to dismantle the central contention that Oswald did it.
Some conspiracies I find plausible based on evidence available:
But even so, popular conspiracies are often based on suspicion of the evidence available rather than evidence for them. I think both the liberal perspective that everything is an accident aka the ruling class doesn't mean to cause bad things to happen and the conspiracist point of view that everything is the result of a conspiracy should be avoided. Speaking to the latter point, the imperialists are powerful but they don't have full-control, in fact, they are often in a state of crisis. Part of their power comes from being able to capitalize on crises.
Giving them too much credit means not knowing their weak points and often discouraging people by making them think that the machine is too powerful to challenge.
As a side-note, we have to go deeper than notions of conspiracy allow us to venture to critiquing core structures and beliefs; that's part of what makes Marxism superior over the guy screeching into a microphone with a tinfoil hat. Sure, he can tell you about aristocratic bloodlines and big gov/corp machinations but what does he have to say to a minimum-wage worker who is being exploited by a small-business owner?
