Modern militaries are almost overburdened with technology - night vision gear, satellite navigation, radios, and other devices designed to allow commanders to give orders and receive data from the field quickly. But what happens if all of that goes away?
It's been known since the first atomic bomb tests that an electro-magnetic pulse (EMP), like the ones released by said bombs, will fry electronics within a relatively close distance. However, you don't need a nuke in order to generate these pulses, and the technology and knowledge needed is largely available
over the Internet. (An EMP was, of course, the weapon used against the Sentinels in the climax of the first Matrix movie, although they don't really cause that funky blue halo of death.)
I can see several scenarios where this sort of weapon could be strategically employed to good effect. For example, a small nation worried about being overwhelmed by the military might of another nation might deploy these weapons along its borders, ensuring that all enemy technology (including planes and guided missiles) that cross its border are rendered harmless in short order. The terrorism possibilities are also endless - an EMP generated as a jet plane was about to land or take off at an airport would be a disaster, and if the device was hidden in a van outside of the airport perimeter the culprits might never be caught. Another possibility would be such a van parked on Wall Street in New York or Bay Street in Toronto - the EMP would fry all the computer systems at major banks and stock markets, potentially crippling the economy without any casualties at all (unless there's somebody with a pacemaker around...).
Fortunately, EMPs can be shielded against, and most military electronics are generally hardened to an extent. But it's simpler to generate a larger EMP than it is to shield against a larger EMP (the shielding adds weight, a problem in high-performace aircraft or portable electronic devices), and civilian computer systems are generally not well-shielded at all.
Just something to keep you awake at night...