The US was caught unaware. What kind of response would you suggest?
Well, I'm not knowledgeable about the various contingencies that have been planned and wargamed incessantly for the past 45 years - but I DO know that the first 30-45 minutes of these scenarios are AUTOMATIC, involve THOUSANDS of officers and men and HUNDREDS of highly visible assets like warplanes, warships, and troop deployments.
None of that happened. Zero, zip, zilch, nada.
Therefore, either our defensive plans no longer exist (improbable), the missile was ours, or it wasn't a missile.