Shaped Charges – Sheet of copper going through 1ft of solid steel (2010) [video]
This is just a few dollars worth of copper and explosives.Attach it to a few tens of dollars worth of drone.  (The cheapest drones are under 10 dollars now).A country could release 1000 of these and direct them at any military target, and they'll do massive amounts of damage at very low costs.Most anti-drone defences can be defeated simply with a redesign of the drone (eg. Use UWB for Comms, and have dual gyros and use a camera for location instead of GPS).I think the only reason we haven't seen this on the battlefield yet is that we haven't yet had a war between the right countries.   But we will.