Rats with bubonic plague. Oh wait, done before.
Aw, hell. It was effective (and there was absolutely zero effort involved), so why not reuse a tried-and-true formula?
Hm... humans with assault rifles... No... Oh!
Moles with coordinate beacons and a radio transmitter to a missile base. Basically, what happens is this:
All allied squads carry a small radio transmitter which transmits at a certain frequency all the time, day and night, constantly. There are moles put into the ground across the potential battlefield. These moles have vibration detectors strapped all over themselves. When a vibration detector detects vibration, it activates a small on-mole computer device. This computer device checks the vibration detected, and stores it as a variable. Then it checks every frequency on a list for a transmission. When a transmission is detected, it accesses an online database for a vibration value attached to the frequency detected, and adds that value to another variable, and checks the rest, doing the same for each transmission detected. If the two numbers (combined vibration value for all frequencies detected and the vibration value detected) don't match up to within a certain range, the mole sends two sets of data to all in-range missile bases. The two sets of data are the vibration detected, and the mole's coordinates within a few meters. A computer at the nearest available missile base first checks to see if the mole's coordinates aren't in a safe zone (such as high-civilian areas), then if the mole isn't in a safe zone, selects the most suitable weapon for the vibration detected, then sends a signal to all the moles within a few kilometers of the transmitting mole's coordinates which starts a timer. The timer coordinates with the detonation of the missile. When the missile detonates, all moles start their timer and coordinate it now with a constantly decreasing variable (residual vibration from the impact), if detected vibration doesn't follow the decreasing variable, it goes through the same process of ordering a missile. If it does follow that pattern, the mole doesn't send information to a missile base, because it would just be hammering empty space.