For any conspiracy to stay a secret, it requires the absolute silence of EVERYONE involved. To pull this off, you have to imagine the number of people involved would be more than a handful.
Now consider this, Watergate started with just TWO people. Look what happened. If you're not familiar with it, Google is your friend. ;-)
Not true! It only requires counter-intelligence, propaganda and dissinformation work. On top of that if you have reasons to stay in and not go public, then you won't go public. How many gov. workers made public CableGate before WikiLeaks? None! How many were there involved? Many. How many made the same secrets public before Snowden? None!
Disinformation can turn the story "a chicken eat a green dinosaur after flying over Africa" into "long time ago dinosaurs lived in Africa. Some of them were the ancestors of today's chicken and eat similar food". You can do anything by altering truth. One tiny missing or extra information can change everything.
For example war in Vietnam started because some Vietnamese ships attacked an US ship (see Gulf of Tonkin Incident). YEars later was found out such thing didn't happened, it was just propaganda to have a reason for war. In Iraq was WMD. That single piece of information made people think they're entitled to start a war.
Information is relative. If you see a guy beating another guy on the street you think "what a bastard". If I tell you he's a convict who escaped from prison and the guy being beaten up is the cop who arrested him, you will be tempted to call police at once. If I tell you the aggressor is actually a poor guy who was beaten previously by the victim with 5 of his buddies, you're thinking "how cool, he caught him alone and gets revenge". One tiny piece of information changes everything you perceive, all meaning changes.
If conspiracies wouldn't work you'd have literally no intelligence services, since they're all made up of many many people. Somebody would betray. And some do. But in the big picture it doesn't matter because intelligence still works.
Problem is of what people think a conspiracy is. Most people think it's some dudes in a dark room planning to screw the rest. Those who do conspiracies are the creme-de-la-creme of the world - smart, with money, power and resources. They know the effects they're after but everybody else beneath them don't, they think they're doing what their doing for various other reasons. For example, the ones who decide know why they start a war. Brainwashed 20yo boys who join the military however think they're going to war to defend the country and their family. If they'd understand the broader picture they'd say "fuck this shit! im not going to risk my life and kill some people so a group of a few can gain geo-political control over an area, and then make more wars in the area and enrage everybody there against my country and its people and turn them from regular fathers, mothers, sons and sisters that mind their business in suicidal fanatics that have nothing to loose because we shelled their whole family while sleeping in their beds".
The greatest scheme pulled by these maniacs at power is they make regular people not even think they exist. As the saying goes, devils greatest achievement was to make people think he doesn't exist. So what happens is this: people instead of realizing in a conflict that the enemy is made by a few in power and simply refuse to fight or do anything against each-other, they think the enemy is the average Joe's in the other camp/country. So each camp thinks the other camp is evil and the enemy and we must fight them. Thing is both camps have to realize the reality and act accordingly. Otherwise the aggressor camp just goes there and steals everything and leaves the locals unable to survive because they destroyed their infrastructure and stole their resources.
If you take the military budgets of all countries and redirect them to solutions for a better life, in 5 years you'd have no famine in the world. Screw that! No! If you'd simply have no wars and aggression you'd achieve that. No need for extra money. Is not the lack of money that's the issue. Is the fact the system is broken on purpose and maintained like this. The moment you are born you enter a brain washing program that lasts your entire life. Everybody brainwashes you because they have been brainwashed too and their parents too, etc. They just propagate crap. Parents teach their kids and enforce on them horrible ideas, concepts and "truths" thinking they're doing the best for their kids, not realizing they destroy them.