Whoever made the sandwich Gavrilo Princip ate on June 28, 1914, has a lot to answer for. Princip had more on his mind than his lunch that day, of course. What the young Bosnian was really thinking about was assassinating Austrian Archduke Franz Ferdinand, who had arrived in Sarajevo that morning for a state visit. Princip missed his first chance to shoot at the heir to the Austro-Hungarian throne and slunk off to have a sandwich.

By pure chance, the royal procession passed directly by the cafe where Princip was eating, and this time he seized his moment, killing Ferdinand – and plunging the continent into World War I.

The nations of Europe – a nest of feuding empires – might well have gone to war no matter what Princip did. But going to war in the way they did, when they did, was certainly the young killer’s doing. Ultimately, 8.5 million military deaths arguably turned on his crime – and the crime turned on his craving for a sandwich.

The opening shots of World War I are an oversized example of what’s known as simplexity – the idea that simple things can be surprisingly complex, and complex things can be deceptively simple. This growing field of study reveals that all manner of phenomena – epidemics, traffic, even politics – move through tiny choke points, seemingly inconsequential junction boxes that may shape the very direction of history.

Of all the people blamed for the Iraq war and the failures of the Bush administration, the name of Major League Baseball Commissioner Bud Selig rarely comes up. But maybe it should. Selig has held his job since 1992, but for several years he was technically the acting commissioner, a team owner merely caretaking the job while searching for a permanent replacement. George W. Bush, then part-owner of the Texas Rangers, was candid about his interest in the position. Selig dithered, Bush gave up and soon decided that politics might be a good alternative career.

The Watergate conspiracy might have unraveled no matter what, but it was a strip of tape on a Watergate building office door that alerted a security guard that burglars were about. Jimmy Carter’s presidency might have crashed and burned anyway, but it was a crashing and burning helicopter in the sands of Iran during a failed rescue of American hostages that may have sealed his loss in 1980. Carter dispatched too few helicopters for so complex a mission, and during the scramble to get away, one of them collided with a troop plane, killing eight soldiers. A few more helicopters might have meant a successful rescue – and a second term for Carter.

The way small causes yield huge effects is itself only one piece of the much grander idea of simplexity, a science that is increasingly being studied at universities and institutes around the world, but nowhere more intensely than at the Santa Fe Institute in New Mexico. The institute was created in 1984 with Murray Gell-Mann – the winner of the 1969 Nobel Prize in physics – as its founding director. No single unified rule governs all complex or simple systems, but there are a few big ones. There’s the concept of phase changes: In the same way that water flips its state from liquid to vapor at 212 degrees Fahrenheit, so too does a stressed geological fault flip from quiescence to quake when it shifts one centimeter too far, or a crowd explodes into a riot when a single bottle is thrown. There’s the concept of relaxation pathways, which models the way rivulets become rivers simply by flowing downhill, or how oceans give up their excess heat by blasting it into the sky as fuel for a hurricane. Understand these simple concepts and you understand the foundations of the larger sciences.

The most powerful of the simplexity concepts, however, is choke points – the keyholes in complex systems that can sometimes shut them down entirely. The streets of New York City are a web of choke points. New York urban planner Sam Schwartz likes to point out that although about 1 million cars enter and leave Manhattan every day, only about 8,000 are in use in Midtown at any one moment. It takes just a few hundred extra cars to gridlock the 8,000, and the 8,000 in turn bring the 1 million to a halt.

The same kind of lucky – or unlucky – breaks have brought Barack Obama and John McCain to the threshold of the Oval Office. If any one of the single-point failures that doomed Al Gore in 2000 (from butterfly ballots to hanging chads to a one-vote loss in the Supreme Court) had not occurred, would Sen. Joe Lieberman, Gore’s running mate, be the Democratic nominee today?

History proceeds in a hard march indifferent to what-if games, but simplexity researchers pay them close mind. Candidates who become too smitten with the power and adulation that swirls around them may want to remind themselves of such things as well. It is great men and women who determine world events – but now and again, it’s sandwiches, too.