The CIA's primary mission became fighting Communism. The first 3/4 of the book lay out how it attempted to accomplish this. The CIA's typical strategy involved identifying a country with the potential to elect a communist government, funding right-wing revolutionaries to overthrow said government, and helping a new government come into power.
In most cases, the new government would be headed by a violent fascist with no respect for law or liberty.
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LBJ got us into Vietnam because of poor intelligence. The Gulf of Tonkin incident was later revealed to be friendly-fire between two US warships, not an attack by the Communists. The error was discovered the same day LBJ ordered retaliatory strikes on Vietnam, but never reported to the legislature or the executive. The mistake was declassified in 2005.
Nixon had a bad habit of ignoring what the CIA told him when it contradicted his policies, even though the CIA was often right and Nixon was often wrong. Sound familiar? Nixon wanted to fight Communism everywhere and wanted the CIA to do it. One of his strongest legacies is the arms race for the cold war. He pressured the CIA for estimates of the Soviet's nuclear arsenal in line with his beliefs, and they delivered, overstating the true numbers until the end of the Cold War.
Carter decided to involve the US in the Soviet-Afghani war, ordering the CIA to funnel arms to the Afghan fighters resisting the Soviet invasion. The shipments went through Pakistani intelligence, which distributed them to the most effective fighters after keeping a fair share of the arms for themselves. The most effective fighters turned out to be the radical Muslims. Some of these fighters later formed the Taliban, of which you may have heard. They took power in Afghanistan in the mid-90's.
Reagan continued Nixon's legacy, using these overstated numbers to bolster the military-industrial complex and further the arms race with the Soviet Union. He approved of the shady arms dealing with Iran and the funding of rebels in Nicaragua and other parts of Central America.
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Bush had little use for the CIA before 9/11, ignoring their warnings that a major terrorist attack on American soil was looming. We all know how that turned out. He had great use for the agency after 9/11, convincing the higher-ups to find intelligence supporting the existence of WMD's in Iraq. We all know how that turned out, too.
It seems like the government wasn't screwing things up enough, so they needed to create a special department for to spend billions to come up with truely spectacular failures, then classify the worst of them.
