What truthers want us to believe is
No, that's only your own perception of how such an operation would have to be setup.
Watching some spy movies doesn't make you an expert in covert/false-flag operations now, does it?
that a fairly large number of people, from many walks of life
No, we don't need the Milkman, or the grocery store vendor, and we don't need 'an army' for an op.
, willingly agreed to kill thousands of their fellow Americans
It is called compartmentalization, like "need to know" but even more sophisticated.
Nobody needs to know that anybody gets killed, not even the guy who carries a bomb in his backpack into the subway,
because he is usually told "this is just an exercise with fake c4" - which he probably regrets to have believed when he either blows himself up with unforunately real C4, or the Squad Team fills him with bullets, asking questions later.
and that every single one of them has kept his or her mouth shut ever since.
The FEW people that actually know what's going on are either so high up the chain that they would never talk,
or if they are not on top, they get either silenced with threats/cash, wont talk cause they are complices, or they get simply ERASED (via an accident, or suicide).
I was in the military and I assure you that it is not made up of mindless robots that would go along with this. Even if you could get every person to follow the orders to carry this out they sure as hell would not keep their mouths shut afterward.
That's why people like you will never learn what it's all about, and only know 10% if anything at all.