Who or what enforces karma?

This is not a mindless force of nature, but it is a force of nature to be sure. It is a force of great power but also great intelligence because it is designed to use information as a means and conveyance of the energy in question. So it is a redirection of energy that takes place across time and can track everything at once and see energy is directed where it needs to go, to render a consequence and readjust and rebalance old wounds and the mistakes of the past. To accomplish this requires precision and an ability to be controlled in a purposeful, logical, and precise way, without error. This is why it is designed to work automatically, so as to remove the possibility of human error, or the error of other beings that may be involved in a particular situation. You know the consequences of human error from experience. It is hardly a match for the laws of physics with respect to their precision, and reliability, and their reproducibility, as well. They can be relied on to repeat the same response over and over and over with an exactitude far exceeding human capability. The Law of Karma is exactly like this because it is a law of physics and metaphysics as well, because it includes much additional information to define the purpose of energy and the information needed to carry out the rebalancing required when the universe is perturbed in some way. Many things that befall humans, especially when they seem to be a perfect comeuppance, when perpetrator suffers the same fate, are attributed to divine intervention, an act of retribution by an angry God meting out justice to the evildoer—this is always an incorrect assumption because it is not what Creator does. Creator's role is to forgive the perpetrator when it is appropriate to do so, to at times step in and redirect things through healing that would otherwise cascade again and again, back and forth, with acts of negativity and then a backlash that triggers another act of negativity, in an almost unending sequence. This can happen when things are left almost entirely to human choice. But that is because of the need for greater learning. This is the task at hand—to grow through enlightenment to better understand the divine path and how to follow the divine way and avoid the trap of karma by creating a negative consequence that will undermine the self inevitably. In this way, the application of justice is built in. It may take an extended period of time for this to play out, but eventually is certain that at some point all will be replaced, repaid, and accounted for, where the punishment will truly fit the crime.