While I disagree withe plant nuke, (I mean really? what kind of spy can assemble and detonate a nuke inside an enemy city?) I would say that dirty bombs are very realistic, as is poisoning water supplies, spreading contagion, and even unleashing a supervirus. these should all be spy actions that kill off some of the city's population initially and create unhealthy conditions for a number of turns afterward. Along the same lines there could be a technology called "The Supervirus" the fist one to gain the technology has the option to unleash it...the effects are devistating. All unarmored units die, all across the world in a number of turns, you recieve 3 hazmat suit marines in each of your cities, you can produce hazmat facilities in your cities that contain the efffects of the disease, and give all units built in that city the hazmat promotion. You then have to take over the whole worls before your raging civilians break into the oval office and linch you.
