If they don't know the force exists, they will have a hard time duplicating the technology because they won't have instruments to measure what is going on. If your technology used electromagnetic waves in a frequency band that others found hard to make images in, they would at least have detectors for the waves and could understand how your devices work. If it is a completely new physical effect, they won't have the ability to measure it, which will slow them down a lot. You might also be able to justify that it wasn't "in the air" and the enemy has already discovered it as well.
Still, you have a perishable advantage. The enemy will figure it out given time. The first downgrade will be when the enemy figures out the general capabilities. They may not know how it works, but if they know what you can see they can blunt the effect. Ask your military strategists to wargame the situation where the enemy knows what your capabilities are. How will they adapt their tactics? Is it still an overwhelming advantage? One example: military bases have sentries to keep intruders out. It is easier to have rings of defense where you catch 90% of the intruders in each ring than catch them all in one ring. 347
They might have guards wandering around randomly looking for intruders. With your sensors, you may be able to evade the guards. The enemy might put more guards on so every spot in one ring is watched all the time. That is still an advantage to you-this costs money and equipment and may withdraw frontline troops-but maybe it is not really big.
Do they have a few very high value assets that you can neutralize if you get up close and personal? Think the Death Star. You need to take them out before the enemy upgrades security. Are you fighting a war of attrition with large volumes of equipment on each side? Do you have the command, control, and computing to make use of it in a mass battle? Trigger a massive battle before they understand what you can do. You can inflict a lethal amount of damage, then mop up what is left.
Since it is a magic device, you can make it small and able to fry itself to stay out of enemy hands. Give each one to a particular person and implant some sensors in them. If they are incapacitated, the device fries itself. This offers a plot line where one of your operatives is captured and the device might not fry itself. If it doesn't, you can play with their efforts to understand and duplicate it. 898