With such capabilities, cruise missiles could be launched from many locations at once and could be detonated on numerous targets at once. The warning available would at best be ambiguous, no different from sensing the presence of shadows in a dark forest.

These terrifying new capabilities on both sides would substantially escalate the chances of a catastrophic accidental use of nuclear weapons during some as yet unforeseen crisis.

How did we get here? The path to the current crisis can be traced to September 2009, when President Barack Obama announced a new approach to American missile defense in Europe against long-range Iranian missiles. It would replace the George W. Bush administration’s plan for what was called Ground-Based Missile Defense sites in Poland and Romania, and Mr. Obama described his new Aegis-based European Phased Adaptive Approach as a “smarter and swifter” defense system. The installations would have a larger number of smaller and slower interceptors that would be guided by Aegis radars, normally used on United States Navy warships.

What Mr. Obama apparently did not know was that none of the radars associated with his program could see long-range Iranian warheads at a long-enough range to launch interceptors. That was a scandalous blunder, ascribable only to a lack of technical information from the Department of Defense and a failure of the White House and State Department policy staffs to adequately vet for the president the implications of the proposed change.

President Obama was probably also not told that the “Aegis ashore” installations in Poland and Romania could be armed quickly with offensive cruise missiles that would pose a threat of an immediate and ferocious attack on European Russia with only short warning.