Supply chain disruption and bad coding
The enemy shields are completely impregnable, but use up a lot of energy. They go through one battery every ten minutes. The batteries, which are based on zero-point energy, use an exotic tech that causes them to explode violently if they get within six feet of each other, so soldiers can't carry multiples.
In order to keep them refueled, the mothership keeps a constant stream of tiny battery drones, roughly the size of a nerf dart, flitting back and forth between the mothership and each soldier.
The shields act as a notification layer between the drones. When an incoming drone is detected, the layer notifies the current drone to head out, then waits for the current drone to acknowledge the signal. Once it does, and begins to head out, the shields open to receive the new drone.
The AI responsible for per-planet calibration of systems did its usual comprehensive analysis of all military technology, flora, and fauna, and gave parameters for drone identification algorithms.
The algorithm, which was optimized by a black-box neural network, boiled down to (roughly):
length == (4.50×10^33 ± 5×10^31),
diameter == (8.00×10^32 ± 5×10^31),
mass == (45000 ± 1000),
speed <= 2.5×10^-41]
(Units are naturally in universal planck dimensions)
There were a couple other conditions, but they ended up getting mostly optimized out or weighted more lightly by the AI, which after some testing found that this set of rules guaranteed the minimum six-nines (99.9999%) of reliability for identifying a drone vs all probable foreign objects.
Unfortunately, it just so happens that your average nerf dart evaluates to
[Cylinder, 4.463×10^33, 6.29×10^31, 45950, 2.21×10^-41]` and just barely gets marked as an incoming battery drone. A less generous speed-metric might block most nerf darts, but it turns out that due to Earth's rough and windy atmosphere, the very lightweight drones have to have a pretty flexible speed range.
When a nerf dart arrives at its shielded target, the following takes place:
- The shields identify it as an incoming drone, and notify the current drone to detach and head out.
- The currently-attached drone gets the message, and obligingly heads back to the mothership to recharge.
- The nerf dart flies through the precisely-provided gap in the shields.
- The alien is harmlessly booped on the sense-bunch by the nerf dart. They glorf at it with their see-stalks, briefly, then get back to work.
- The shield notifies the mothership that it's getting low on power, and asks for a battery.
- Another drone battery shows up.
- The shield lets the nerf dart know that it needs to head out.
- The nerf dart doesn't respond.
- The shield idles, and doesn't let the new drone in until the old drone leaves.
Since the nerf dart is never going to respond to the shield's signal, the shield is never going to let in the new drone to recharge. A short amount of time later, the shield deactivates for lack of power and the alien:
- Is shot with conventional weapons
- Suffocates on Earth's atmosphere
- Suffers some other unpleasant, shieldless fate.