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."