I was interested in hearing about events in history in which an individual, or relatively small group of individuals through a violent act made a major historical impact. Examples I’m thinking of are things like the 19(?) highjackers responsible for 9/11 or the assassination of JFK by Lee Harvey Oswald. I’m thinking more of incidences that were a single act rather than a series of acts. The examples can be from any time or place.
It’s an interesting question, I think, because science fiction (as seen in Star Trek’s Prime Directive, etc.) leads us to believe that a small act by an individual can irrovacably change history. Witness is it City on the Edge of Forever? – the famous Star Trek episode starring Joan Collins, where a whacked-out McCoy goes through a portal and BOOM then Enterprise is gone and the course of humanity has changed forever.
Timeline by Michael Crichton (I know, I know) addressed this very point and he argues that really, there’s very little one can do that would have that large of an impact. Go on a shooting spree at the Super Bowl? Hm, just try to get in with a gun. And so on. We think we have the power to change things in a major way, but unless it’s someone who already has a lot of power, it ain’t gonna happen.
It doesn’t answer what you asked, I guess – but it’s enjoyable thinking about. 
Although no one individual can receive the blame by himself/herself for the Hiroshima A-bomb, that event certainly had its historic significance.
Political assassinations, too numerous to detail, all have had their history-changing implications.
Knowing how things did change after some violent events doesn’t give us many clues to how things might have changed were it not for those same kinds of events. I mean to suggest the untimely losses of potential movers and shakers due to accidents and mishaps. It’s hard to document might have beens.
Mark David Chapman’s assassination of John Lennon had a profound affect on music and history
Such as? While his assassination certainly had a large emotional impact, I’m hard pressed to see how it made a significant historical impact.
Marc
Edited for speling…ha ha.
The assassination of Abraham Lincoln by John W. Boothe had a signifcant historical impact. Reconstruction might have gone easier for the south had Lincoln been around.
Marc
I wasn’t alive at the time, but from what I gather the Watergate scandal fairly well brought about modern mass media as the watchdog of government, and the warts-and-all approach to reporting. And while Nixon was president, I would still have to say that that’s more something that one man did, rather than something that “The President” did.
Lee Harvey Oswald no doubt changed US foreign policy significantly.
Gavrilo Princip precipitated World War I by assassinating Archduke Ferdinand, and since the result of WWI was pretty much the direct cause of World War II, I would have to say that he more than anybody else in history changed the world.
It wasn’t particulary violent, but how about the person who poisoned Tylenol with cyanide in 1982? Pill bottles have never been the same since then.
Dammit, I was gonna suggest that.
To be fair, the political situation in Europe was already pretty dire. His action just happened to be the straw that broke the camel’s back. If he hadn’t done it, it’s pretty likely that some other event would have started the war.
But then again, that appears to be what the OP is looking for: a relatively minor act of violence that ended up having a profound impact on history.
A small group of senators assassinated Julius Caesar. At the time of his death, Caesar was planning an attack on the Parthian empire. Being who he was, he probably would have succeeded in conquering it, leaving Rome in posession of the East as far as India. The entire history of the region would be different, i.e. probably no Mohammed, perhaps no Jewish revolt either.
Princip is definitely the best example. There might have been war without him, but not at that particular time and possibly not so widespread.
Leon Czolgosz’s shooting William McKinley brought Theodore Roosevelt to office and changed things from a do-little president to an activist.
The assassination attempt on president-elect Franklin D. Roosevelt would have made all sorts of changes if it had succeeded. It’s highly doubful John Nance Gardner would have pushed for the New Deal policies.
Under this, I would submit Mahatmah Ghandi’s nonviolent protests to secure Indian independence from the British. Granted, it’s not the biggest thing to impact the Western world, but it did liberate an entire nation and teach others that nonviolent protest can work.
Tripler
Violently? A bare minimum of physical violence, but a whole shitload of political violence.