A video of the SDI test

Video shows moment US intercepts target ICBM

Look closely and you see no decoys  coming from the target missile.  Now, I might be giving away secrets, but the Di Kim Son does have techies who are not so dim. A decoy warhead is just a traffic cone, put  three or four of them up and release them from the warhead in mid-flight.  Mylar balloons, for example, with a tiny bit of air will inflate in the vacuum and look like a warhead.

Now a law professor like Glenn Reynolds does not get the problem, in fact, he misses a lot of legal issues. But there is no way the defense in this game wins, there is no cost effective way to deal with decoys.  In fact, I think I wrote the only targeting algorithm that dealt with decoys successfully (second test in the 80s was me). But I had already knew the likely deployment pattern.  Beyond that, not much has been working against the decoy problem.