You've no doubt heard of the butterfly effect, wherein the flapping of a butterfly's wings can, over the course of time, distance, and accumulated consequences, produce a tornado. In our political moment, Jeff Sessions is that butterfly.

Or more precisely, it was President Trump's decision to make Sessions his attorney general, which seemed perfectly reasonable at the time, that could well determine the entire course of his presidency and quite possibly its ignominious end.

Yet this is one of the rare cases where Trump can't be blamed for his mistake. In fact, when he appointed Sessions it was one of the rare moves he made that was exactly what a typical Republican president would do. Sessions was the first member of the Senate to endorse Trump, so the plum job was in part a reward for his loyalty. As a former federal prosecutor, state attorney general, and member of the Judiciary Committee, he was certainly qualified. While he had some, ahem, colorful history on matters of race, as a member of the Senate it might have been assumed that he'd have a relatively easy time winning confirmation from his colleagues (though in fact he wound up getting the vote of only one Democrat, West Virginia's Joe Manchin).

And perhaps most importantly, since he was from Alabama, a state Trump won by 28 points and where there is not a single Democratic statewide elected official, there seemed to be no political risk involved. The Republican governor would appoint Sessions' replacement, and then that person, or at worst a different Republican, would win the special election to serve out the rest of Sessions' term.

Of course, at the time no one figured on Roy Moore not only running but winning the primary, then turning out to have had an attraction to girls on the south side of the voting age.

So now that Democrat Doug Jones is going to be the next senator from Alabama, not only might the Republican tax bill be in peril (if they can't push it through before Jones is sworn in), the chance that Democrats will take control of the Senate in the 2018 elections has increased dramatically.

Democrats will need a net gain of two seats instead of three, which might not seem like a big difference until you consider that until recently only two Republican-held seats were considered truly vulnerable, those in Arizona and Nevada. It's entirely possible that all the Democratic incumbents hold their seats, and those two seats fall. Before this Tuesday's special election, that would have meant a 50-50 Senate, with Mike Pence breaking any ties — far from an optimal situation for Republicans, but one they could have lived with.

But now, the same electoral outcome produces a Senate controlled 51-49 by Democrats. That would mean they'd have the votes to kill any piece of Republican legislation, they'd control all the committees, and they'd have subpoena power, which means they could start a hundred investigations into the Trump administration's activities. In many ways, they would be able to grind the Trump presidency to a halt.

But that's only half the story of the flapping of Sessions' butterfly wings. The other half is the Russia probe.

Once again, the problems the Sessions appointment have created were nothing Trump could have foreseen. He didn't know that Sessions was going to mislead Congress about his meetings with the Russian ambassador, then decide that his own involvement in the issue required him to recuse himself from the matter. That enraged Trump — as he told The New York Times, "Sessions should have never recused himself, and if he was going to recuse himself, he should have told me before he took the job and I would have picked somebody else."

But because Sessions recused himself, the authority fell to Deputy Attorney General Rod Rosenstein, who made the decision to appoint a special counsel and chose Robert Mueller for the job. Almost before Trump knew what was happening, he had a seasoned, professional, and apparently determined prosecutor on his tail, one who will be very difficult to remove (though that doesn't mean Trump won't try).

When Trump appointed Sessions, he probably assumed he was getting an attorney general who was loyal to him and would protect him, which is clearly what he wanted. Instead, he got someone who has made himself unable to do so on the scandal that most threatens his presidency.

It's not an exaggeration to say that had Trump appointed someone different to be attorney general, there might be no special counsel at all, no indictments, no former aides arranging deals to tell what they know, and a scandal that might have gone nowhere. Yes, there are still congressional investigations, but those have been slow and toothless (so far, anyway).

And so Sessions, who wanted nothing more than to lock up some hippies and restrict some civil rights, could be the one whose appointment winds up causing both a political and a legal crisis for the president whose side he rallied to all those months ago. It's a lesson in what makes politics so complex and difficult to predict: Even a decision that seems to carry little or no risk can set off a chain of events that ultimately results in catastrophe. Which is why Donald Trump will forever curse the day he made Jeff Sessions attorney general.