Shaped Charges – Sheet of copper going through 1ft of solid steel (2010) [video]
This is just a few dollars worth of copper and explosives.<p>Attach it to a few tens of dollars worth of drone.  (The cheapest drones are under 10 dollars now).<p>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.<p>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).<p>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.