ROME, Aug. 22 -- The arithmetic of air defence has been running the wrong way for years. A one-way attack drone costs a few tens of thousands of dollars. The missile fired to stop it costs a million. Italy has just placed an order aimed squarely at that gap.

The State Department approved a possible Foreign Military Sale to Italy on Friday of 5,031 Guidance Sections Single Variant Air-to-Air Advanced Precision Kill Weapon System-II, designated WGU 59B/B, at an estimated total cost of $364 million. The principal contractor is BAE Systems, at Nashua, New Hampshire.

What Italy is buying is not a missile. APKWS is a guidance kit that bolts onto an existing unguided 70mm rocket, turning a dumb tube of explosive into a laser-guided one for a f...