In the grand scale of things, not much changes when a single dude decides he'd rather be a Communist than an American, or vice versa. The balance of power is still basically the same, right?
But every balance has a tipping point, and sometimes all it takes is one turncoat to change the course of history. Like ...
5Lee Harvey Oswald
The guy who killed President Kennedy, unless you believe the conspiracy theories.
Changed Everything By:
Not long before the Kennedy assassination, Lee Harvey Oswald defected from the Soviet Union to the United States (after first defecting from the USA to the Soviet Union).
If he'd decided to stay there, or not been allowed to leave, JFK would have lived and some alternate version of history would have played out.
Lee Harvey Oswald wasn't a crazed loner -- he didn't shoot JFK because he thought the White House was beaming mind control waves into his brain. He had an actual beef with the American government, and by that we mean he was kind of a communist sympathizer. So much so that, four years before he climbed to the sixth floor of a book depository and became one of the most hated men in American history, he flipped America the middle finger by defecting to the Soviet Union. And since he was just a random, whiny douche at that point, America didn't put up a big fight to keep him. Would you?
Unfortunately for the whole world, life in Soviet Russia wasn't as exciting as Cold War propaganda posters let on. By 1959, the USSR was more like the bleak, gray, impoverished, always-snowing suicide trap you see in photos like these:
Those aren't hats.
Oswald had hoped to study at Moscow University, but the Soviets had a job opening for a lathe operator in Minsk, and apparently, in Soviet Russia, the job chose you. (Sorry.) The idealistic Oswald must have been pretty disappointed by what he saw. So did he change his mind, having a revelation that maybe the American way was better? Eh, not exactly.
As Oswald himself wrote in his journal: "I am starting to reconsider my desire about staying. The work is drab, the money I get has nowhere to be spent. No nightclubs or bowling alleys, no places of recreation except the trade union dances. I have had enough."
"Ladies and gentlemen, please welcome ... Lee Oswald and the Trade Union Dancers!"
Yep, it was the lack of bowling alleys and nightclubs that drove him back to the USA, and back onto the path that would lead him to killing a president.
By all rights, at that point the USA should have told Oswald to go fly a kite, trapping him in Russia where he could never hurt a pinko fly, much less the president. Instead, the United States let the wayward dipshit come home, presumably after a hearty round of "Told you so," and the USSR let him go, presumably because his lathe machine operating skills sucked.
So Oswald comes home and goes nuts, right? Not at first. First he tries to defect to Cuba, since everyone knows the second time is the charm. But this time the U.S. wasn't having it. A few weeks later, President Kennedy decided to visit the home town of a twice-jaded trigger-happy Communist newly enraged at being told he was trapped in America forever, and conspiracy theories notwithstanding, the rest is history.
"Ironically, I am actually three people."
A Soviet spy so shitty at spying that he wasn't allowed to spy any more.
Changed Everything By:
Raiding the Soviet's document archives and turning everything over to the West, giving them a huge advantage in the Cold War.
And having a mustache that redefined facial hair for fast food restaurant managers worldwide.
Vasili Mitrokhin began his promising career with the Soviet Motherland as a spy for the KGB. However, in a plot befitting a Steve Carell comedy, he bungled a mission and his employers gave him a new permanent assignment: head of the KGB archives. In other words, they punished him by making him a librarian.
It wasn't this humiliation that turned Mitrokhin against the USSR, but rather what he found when he got there. Access to the Soviet Union's most secret of secret documents revealed the terrifying extent of the nation's efforts to oppress and terrify its own people. And though he'd been losing faith in Stalin's brand of communism for some time, details of events like the crushing of the Prague uprising clinched the deal. The only question was, what kind of damage could one man do to the Soviet Union?
"Let's see how they like it when I steal their most beloved recipes!"
Quite a bit, if that man was left alone in a room full of top-secret Soviet government files and had a lot of free time.
Day by day, Mitrokhin made copies by hand of top-secret Soviet reports and smuggled them back to his summer cabin. Of course, the problem of what to do with them remained. His life-endangering efforts to contact the CIA fell on deaf ears, so he instead attempted to defect to the British. Once the value of the Mitrokhin archive fully came to light, the Brits politely invited Mitrokhin and his family to come stay with them for tea and crumpets and any paperwork he wanted to share. What he brought with him turned out to be "the most complete and extensive intelligence ever received from any source."
Mitrokhin's treasure trove of top-secret intelligence identified scores of Russian spies in frighteningly high-rank government and media positions. It also exposed unknown political alliances the USSR had with other countries and details of secret military operations (including the plot to assassinate Yugoslavian leader Josip Tito). It even included information on Soviet propaganda campaigns within the USA to perpetuate various conspiracy theories (i.e. the JFK assassination, J. Edgar Hoover being gay, the CIA killing Martin Luther King Jr.) and to foster the rise of the KKK. They were like the Gossip Girls of international espionage.
But, man, could they ever dance.
Though the Soviet Union was already beginning to implode by the time Mitrokhin got his information out, it nevertheless destroyed any leverage the USSR might have had over the West by effectively ending their entire spy program. Who ever said librarians had dead-end careers?