Bob was having a horrible day. Over the past few hours, his plans, his dreams, had been turned upside down. Six hours ago, a colony ship bound for Alpha Centauri had left Pluto outpost. Four hours later, it had passed through the Oort cloud’s border. One hour after that, they received a transmission from the opposite side of the solar system, apparently from the colony ship.
Bob, the President of the Solar system, had dreamed of being remembered as the man who pioneered the far reaches of space. Now, due to some unseen force, the Human race may never extend its reach beyond the rays of Sol.
He suddenly noticed a man, sitting in the guest’s chair. Startled, Bob jerked himself out of his reclining chair. “Who are you?”
“Forgive my sudden entry, Mr. President. I am pleased to tell you that your race, while possessing sufficient technology to colonize other solar systems, is not culturally advanced enough to be allowed outside of your system of origin.”
“Says who!”
“So states the Federated League of Races. The Treaty of Shigonto, the basis for the League’s foundation, clearly states that no race which is likely to begin wars is to be allowed into the position were it may do so. Humanity has been determined to be warlike, therefore it cannot be allowed to propagate itself beyond this solar system.”
Bob had been growing more and more angry during this speech. He jumped at the opportunity to speak. “A blockade is an act of war, you know. Such things have been forgiven in our past, but the blockade must be removed.”
“Tell me, Human, how do you think any attack by your forces against an enemy capable of erecting such a barrier?”
Bob was defeated on that point, but he was not going to give up yet. “That wasn’t my point. My point was that you don’t approve of war, but you committed an act of war.”
The man smiled. “The solar system you attempted to send your ship to is inhabited. The ship held what constituted an invasion force, as I doubt that you would have allowed the peaceful, plantlike people there to survive. Your colonists would have cut them down, set them on fire, or buried them with buildings.”
“We didn’t know that it was inhabited.”
“No, you didn’t. and that’s yet another reason you cannot be trusted with our universe. You could have colonized and destroyed their entire species before it was noticed that they were just like you.”
Bob had run out of arguments. “What do you want us to do?”
“I would see this decadent species exterminated, but as much as I would like to, my employers’ collective approval of the Treaty of Shigonto forbids me to do so. The barrier will be lifted when the Human race has proved itself by not coming into conflict with itself, on an individual basis or on any scale. Conflict is defined by the use of force to harm someone. If on any one day this happens, my employers will come and announce the quarantine is ended.”
The man picked up his black umbrella and left.

Ten billion years passed. Humanity had spent itself in a grand war that had destroyed three of the four rocky planets, made uninhabitable every colonized moon, and severely destabilized the Sun. before the Sun went nova, the Aliens that had confined Humans so long landed on Earth, the last surviving planet. After many tests, they came to a conclusion. Humans were not the victims of any outside cause which made them aggressive, they were just a bad crop.
The man with the black umbrella could have told them that.