One example of these uncontrollable things that people use in battle is nuclear energy. It was supposed to be used as a new, powerful resource of energy that might be better than fossil fuels or natural gas, but ever since Einstein made the famous equation of 'e=mc2', the world now knew that nuclear energy can be used as a much more powerful war machine than any other in its time: the nuclear warhead, or the nuke. It was used by the USA to annihilate Hiroshima and Nagasaki, then used in the Cold War to show off power, and now used by North Korea to threaten the USA. A single nuke can decimate an entire city, so it's also no wonder that scientists predict that nuclear war will cause the apocalypse.
Another thing that humans can't learn to control yet is time travel. Of course, humans haven't known how to make time travel possible, and I think it'd better stay that way. Imagine, if time travel was accessible, then humans would recklessly travel through time, carelessly changing the course of history with dire consequences. In time travel, you must be really careful of what you do, because if you change a tiny, seemingly insignificant thing, it might result in the course of history being changed forever and might even result in our extinction. I won't torture you with all the physics and rules about time travel, but if time travel fell into the hands of terrorists, God knows what might happen to us. They might go back in time to rule different eras, and change our fates. In short, it might not be wise to have the power over time travel.