MOSCOW — As US and allied officials celebrated the opening of a long-awaited missile defense system in Europe, the reaction in Moscow on Thursday was darker: a public discussion of how nuclear war might play out in Europe and the prospect that Romania, the host nation for the US-built system, might be reduced to “smoking ruins.”

“We have been saying right from when this story started that our experts are convinced that the deployment of the ABM system poses a certain threat to the Russian Federation,” the Kremlin spokesman, Dmitri S. Peskov, told reporters in a conference call. “Measures are being taken to ensure the necessary level of security for Russia.”

The United States has asserted that the antiballistic missile system would protect only against “rogue” states, particularly Iran, and provide no protection for either Europe or the United States from Russia’s far larger arsenal of nuclear missiles. The North Atlantic Treaty Organization site will be controlled by a US officer.

The system, called Aegis Ashore, was essentially transferred from a seaborne launchpad onto land in Romania, at the Deveselu air base. The United States planned to break ground Friday on a second site, in Poland, that should be completed in 2018. But a deputy US defense secretary, Robert Work, reiterated Thursday there are “no plans at all” to strengthen this missile umbrella to protect against Russia.

While the United States says it has no Tomahawk missiles at the site in Romania, the launchpad violates a 1987 treaty intended to take the superpowers off their hair-trigger nuclear alert, the Intermediate-Range Nuclear Forces Treaty, by banning land-based cruise and medium-range missiles with a range from 300 to 3,400 miles.

President Vladimir Putin has warned that a US antimissile deployment in Eastern Europe could prompt Russia to withdraw from the treaty. The United States accused Russia of violating the treaty last year by failing to declare the true range of two missile types.