(Warning: The following contains one or two fairly substantial — but relatively inconsequential — SPOILERS for 'Captain America: Civil War.' Proceed with whatever level of caution your innate sense of SPOILER-aversion, and amount of having actually seen the movie, suggests is wise.)
Now, unless you adhere to a very particular theory of history, there's a pretty good chance that you don't see the actions of one single individual as being capable of starting an entire war. For all that we can change the world with the decisions we make, there tend to be so many other individuals making decisions around us that — even were we high ranking generals, or the President — the ability to single-handedly start a war would likely be beyond us. Without support staff, governmental oversight, and a whole lot of international culpability, it likely simply couldn't happen.
A shame, then, that no one seems to have told that to Iron Man.
It Seems That Tony Stark's Personal Problems May Have Directly Caused Marvel's Civil War
(Note: This is where those aforementioned plot SPOILERS really start to kick in, albeit largely in trailer-teased form.)
So, here's the thing about Captain America: Civil War, and its central superhero-v-superhero plot-line: Were it not for Tony Stark, it would never have happened.
Now, to be clear, I don't mean that in a butterfly effect, Iron Man started the slide down the slippery villain-filled slope that is the Marvel Cinematic Universe sort of way (though that's also a valid point). Instead, I'm talking about something distinctly down to earth and straightforward: Had Tony Stark not chosen to back the Sokovia Accords (the UN-sanctioned restrictions on the activities of superheroes), then there would never have been a superhero civil war at all.
After all, had Tony decided that he disagreed with the Accords on principle, and teamed up with Cap to oppose them, then there's a pretty solid chance that the vast majority of the Avengers would have done the same. Sure, that might have led to an entirely different conflict — one between the Avengers and the US government — but it would also have made Captain America: Civil War a very different movie with a far more misleading title.
How do those aforementioned personal problems come into it, though?
It Sounds As Though Tony Stark Mainly Supported The Accords Because Of One Big Personal Problem
Specifically, the fact that, as Captain America: Civil War suggests (and that, being a little more SPOILER-ific than what was discussed above, is SPOILER-protected below)...
...Pepper Potts has, it seems, broken up with Tony a short while before the events of Captain America: Civil War. Which, as he reveals during the course of Civil War, has had a direct and seemingly decisive influence on his decision to back the Sokovia Accords: "I don't want to lose her. I thought the Accords could split the difference."
In other words? Tony's failure to keep his promises to Pepper — the implication seems to be that he was unable to walk away from super-heroism as he told her he would, and she ultimately called him out on it — was a major factor in his decision to choose government oversight over simply trusting in his friends, a decision which ultimately led to Civil War.
Without that particular personal issue of Tony's, then? Well, we may not have seen Civil War happen at all. Or, y'know, it may have happened anyway, with the beating of a metaphorical butterfly's wings causing a series of seemingly unrelated events to bring an entirely different hero to the fore, thus creating a very different — yet strangely similar — Civil War.
Or, y'know, we might have just watched Cap and Iron Man beat up faithful government agents for 20 minutes or so, before solving the movie's mysteries in record time. Sadly, however — for Tony in particular — we'll never know.