"The difference between fact and fiction is that fiction must be believable."--Mark Twain
When looking back at history it's hard to find too many examples of successful conspiracies that didn't accomplish their goals fairly quickly. Often conspiracy theories have a greater effect on history than conspiracies themselves. But of course that makes actual conspiracies that lasted over many years and ended up accomplishing what they wanted all the more interesting.
As I was reading The Sleepwalkers, a book about Europe's slide into World War I, I came across the most spectacularly effective conspiracy. Back around the start of the middle of the 19th century Serbia got its independence from the Ottoman Empire in the Congress of Berlin. It fought several wars to gain much of the territory it felt was naturally part of Serbia.
During this time there were many groups in Serbia devoted to Serbian expansion. The main one was the Black Hand, a group formed by Dragutin Dimitrijević, better known as Apis, who had helped assassinate the previous king of Serbia. Many members of the Serbian government and military were members though notably not the prime minister. At this point all the Serbs under Ottoman rule had been freed, so there remained only to liberate the Serbs living in Bosnia under the rule of Austria-Hungary. Well, some of those people were Bosnians or Croats but if you're a Serbian nationalist at this time Croats are just religiously confused Serbs, right? And who cares about the Muslims.
So the Black Hand went about trying to figure out how to go about getting Bosnia into Serbia. It would probably take a war but Russia was pretty firmly on Serbia's side and the whole international situation looked pretty good from the Serbian perspective. Read The Sleepwalkers for more details.
Eventually Apis seized on the idea of assassinating Franz Ferdinand. Partially this was a way of pushing Serbia and Austria-Hungary into war but if not Franz Ferdinand had been very keen on granting the empire's Slavs more rights and thereby stunting the Bosnian's and Croat's desire for independence.
And so weapons from Serbia made their way to Sarajevo where they were successfully used to kill the archduke. This started a war, just as intended. And the system of alliances worked out well for Serbia, as intended. And after the war Serbia was essentially given all the territory the Black Hand wanted it to have. So in a way this conspiracy succeeded spectacularly.
But they didn't live to see it. During WWI the leadership was put on trial over trumped up charges and mostly executed. So they didn't live to see their success, but they did see the horrendous devastation their war had brought to Serbia.