In popular culture on my Earth-like planet, alien-abduction stories, anal probing and conspiracy-theories are well-known but largely considered by mainstream culture to be delusions or fanciful ideas taken up by the imaginative, the lonely and the desperate.
Tales often have the common feature describing sadistic experiments, transplants eavesdropping on our thoughts and anal probing.
Of course it (mostly) turns out to be true.
One fine day, an alien fleet turns up, takes over all the communication channels, disables the military-infrastructure of the world instantly by computer virus and demands that we surrender on pain of planetary destruction. After brief attempts at negotiation, threats, a few feeble attempts at shows of force, humanity complies.
The aliens land in cities all over the world and demand that everyone submit to one specific test - an anal probe. When asked why, all they will answer is - "It is necessary for the survival of both our species."
What reason could a technologically and militarily superior race have for wanting to probe everyone's anus?
Whilst any brilliantly argued answer (with a valid frame-challenge) may be accepted, in an ideal world these conditions will hold:
The humans will not immediatley understand why what happened has happened, but as the aliens depart after the "event", they are left with a powerfull and lasting racial memory.
Humans, on moving out into the galaxy, encountering other races, forming alliances and becoming part of the larger galactic civilisation - stumble upon the answer to the riddle and choose to seek redress through the Galactic Courts.
The Galactic Courts have "Sapient Individual's Rights" roughly equivalent to The Geneva conventions.
The Aliens put-up no defence, the humans win outright and recieve substantial compensation.
The Aliens continue as before.
As per JBH's comment, the implication is that comments should have been left asking how better to define the question before any answers were published. My fault for asking a badly constructed question, your fault for answering rather than commenting to improve the question. Your answer is technically still valid, and you may still edit it. None of this implies that an acceptable answer has not already been written, but I need to keep the question open the customary 24 hours before I award that.