Are conspiracy theorists all lunatics?
There have been too many 'unbelievable' conspiracies, the meaner the more unbelievable, to now discredit people who come up with wild theories about false flag attacks being set up by secret services, or murderous terrorist attacks being encouraged by them, 'justifying' an invasion or militairy actions for a regime change in another country.

Conspiracies for war and genocide
Many think that conspiracy theories, in which a government, a secret service, or a large company plays a role, are an invention of the simple minded, or the less educated who are unable to cope with the harsh reality of real terrorists attacks.
The reality however is, that those who think so, may themselves lack some knowledge of history, since conspiracies of governments, secret services and large companies have been proven to exist as long as mankind. Nothing assures us that conspiracies belong to the past.
Regarding modern conspiracy theories, many say that a conspiracy in which thousands of people participate is impossible. They're mistaken since history proves that even the minds of hundreds of thousands of ordinairy citizens can be manipulated in a way that makes them 'look the other way', or makes them accept a dirty job must be done now and then to free the world from a bigger evil.

Nazi Herman Göring at his trial:
"Of course the people don't want war. But after all, it's the leaders of the country who determine the policy, and it's always a simple matter to drag the people along whether it's a democracy, a fascist dictatorship, or a parliament, or a communist dictatorship. Voice or no voice, the people can always be brought to the bidding of the leaders. That is easy. All you have to do is tell them they are being attacked, and denounce the pacifists for lack of patriotism, and exposing the country to greater danger.

A conspiracy later known as the Nazi Holocaust
1933 - 1945.  'Die Endlösung' (The Final Solution), was an invention of a chosen government, and impossible without the cooperation of large companies, factories, the railways, police officers and ordinary citizens. They may all have had their own reasons for their participation, but a conspiracy it was, since it was a secret, and consequently denied. It was the plan to murder all Jews in the Nazi-occupied territories or fatally exhaust them by slave labor. Together with them, communists, handicapped, gipsies, homosexuals, Jehova Witnesses, thiefs and those who tried to obstruct the plan were destined for extermination.

'The Destruction of the European Jews', (1985)
by Raul Hilberg.
The Holocaust, Wikipedia

A conspiracy to start a global war
1939 August 22. Adolf Hitler told his generals:
"I shall give a propaganda reason for starting the war, whether it is plausible or not. The victor will not be asked whether he told the truth."

1939 August 31.  In a German secret operation, called 'the Gleiwitz incident' the german Intelligence organisation 'Abwehr' and the SS (SchutzStaffel) took the life of a German prisoner, Franciszek Honick, dressed him like a Polish soldier, and used his dead body as 'proof' of a Polish attack on a German radiostation close to the Polish border. Other Germans, dressed like Polish soldiers, took the radiostation, and broadcasted a short anti-German message, to make it all look real. Some prisoners from the Dachau concentration camp were kept by hand, for in case they, or their bodies were needed to make it all look more convincing. At the same time, houses were torched. A total of 21 incidents were made to look like provocations by the Polish.

Next morning: invasion of Poland

1939 September 1.
  the German Army invaded Poland. Chansellor Adolf Hitler pretended the invasion was provoked with the following words spoken before the Reichstag:
"Seit 5uhr45 wird zurückgeschossen!"

("Since 5.45am we have been returning the fire".)
Hitler needed this lie to get the German people behind his plans for the expansion of German controled territory, by war.

The Gleiwitz incident

1939 September 1. Hitler's speech
"... This night for the first time Polish regular soldiers fired on our territory. Since 5.45 A.M. we have been returning the fire, and from now on bombs will be met by bombs. Whoever fight with poison gas will be fought with poison gas. Whoever departs from the rules of humane warfare can only expect that we shall do the same. I will continue this struggle, no matter against whom, until the safety of the Reich and its rights are secured....."
Read the whole speech (translated to english)

IBM and the Third Reich.
1937. German reward for Thomas J. Watson.
"Watson personally approved and spearheaded IBM's strategic technological relationship with the Third Reich.[4] In this relationship, IBM helped make Nazi Germany more efficient; as it did its enemies Britain, Soviet Russia, and the United States of America, where IBM did substantial business as well. In 1937, Watson received the Eagle with Star medal from Germany for the help that IBM subsidiary Dehomag (Deutsche Hollerith-Maschinen Gesellschaft mbH) and its punchcard machines provided the Nazi regime for tabulating census data. After the outbreak of The Second World War, Watson returned the medal, and the German government tried to take ownership of the Dehomag operation"

The Iran-Contra conspiracy of 1998
One of the proven and recent mayor conspiracies is the Iran-Contra affair. The US military secretly sold weapons to Iran, while officially there was a ban on weapon sales to Iran, since Iran held US diplomats hostage. To make it even worse, part of the money earned with the arms trade was secretly spent on 'the contras', rebels who were fighting the democratically chosen government of Nicaragua.
The investigation of the Iran-Contra scandal brought to light, that there was a secret government at work within the USA, within its military and its Central Intelligence Agency, that needs a naive president in order to secretly carry out its illegal operations, or a president that was an excellent actor, pretending he didn't know a thing about what was going on.
Watch the documentary:
Cover up: Behind the Iran-Contra Affair (1998)
Google video, 1hour and 12 minutes.