John Kerry told the Senate Foreign Relations Committee on Tuesday that "Russian provocateurs" had infiltrated eastern Ukraine in order to foment "an illegal and illegitimate effort to destabilize a sovereign state and create a contrived crisis." Also on Tuesday, the Pentagon announced steep cuts to U.S. nuclear forces, four years ahead of schedule, in accordance with the 2010 New Start treaty with Russia.

At this point in Barack Obama's Presidency we should be used to the mental whiplash. But we still feel concussed. 

So let's slow down and follow the thread. Russia has seized Crimea and has 50,000 troops as a potential invasion force on the border with eastern Ukraine. The Kremlin is also abrogating the 1994 Budapest Memorandum, in which Kiev agreed to give up its nuclear arsenal—at the time the third largest in the world—in exchange for guarantees of its territorial integrity from Russia, the U.S. and U.K. That memorandum has now proved to be as much of a scrap of paper to the Kremlin as Belgium's neutrality was to Berlin in the summer of 1914.

The Kremlin is also violating the 1987 Intermediate-range Nuclear Forces Treaty, which bans the testing, production and possession of nuclear missiles with a range between 310 and 3,400 miles. Russia has tested at least three missiles—the R-500 cruise missile, the RS-26 ballistic missile and the Iskander-M semi-ballistic missile—that run afoul of the proscribed range limits.

The Obama Administration has suspected for years that Vladimir Putin was violating the INF Treaty, which supporters hail as the triumph of arms control. The Russians were boasting of their new missile capabilities in open-source literature as far back as 2007. Yet as defense analysts Keith Payne and Mark Schneider noted in these pages in February, "since 2009, the current administration's unclassified arms-control compliance reports to Congress have been mum on the Russian INF Treaty noncompliance."

At a minimum, Congress should call on Rose Gottemoeller, confirmed last month as under secretary of state for arms control over strenuous objections from Florida Senator Marco Rubio, to explain what the Administration knew, and what it disclosed, about Moscow's INF violations when she negotiated New Start.

Ms. Gottemoeller has been publicly noncommittal on this point, perhaps because she knew New Start would never have won a two-thirds Senate majority if Russia's INF cheating had been widely known. The episode reminds us of why people like former Arizona Senator Jon Kyl were right to oppose the ratification of New Start.

Which brings us to the Administration's announcement on cutting U.S. nuclear forces to levels specified by New Start four years before the treaty's 2018 compliance deadline. The news comes a few days after Hans Kristensen of the Federation of American Scientists reported that "Russia has increased its counted deployed strategic nuclear forces over the past six months." Yet at the same time America's stockpile of warheads and launchers has declined.

Mr. Obama has dismissed Russia as a regional power, but he is maneuvering the U.S. closer to a position of absolute nuclear inferiority to Russia. The imbalance becomes even worse when one counts tactical nuclear weapons, where Russia has a four-to-one numerical advantage over the U.S.

To the surprise of defense analysts, the Pentagon will make the sharpest cuts in the submarine and bomber legs of the nuclear triad, while mostly preserving the silo-based Minuteman ICBMs. This means that the U.S. will maintain a stationary, and vulnerable, nuclear force on the ground while largely dismantling what remains of our second-strike capability at sea and in the air. A crucial part of deterrence is convincing an adversary that you can survive a first strike. It does not help U.S. security to dismantle the most survivable part of the U.S. arsenal.

It's fashionable in the West to dismiss this as "Cold War thinking," but it appears that Vladimir Putin hasn't given up on such thinking or he wouldn't be investing in new nuclear delivery systems.

Cold War or no, recent events are providing daily reminders that the great-power rivalries of previous centuries are far from over. They have also offered the grim lesson that nations that forsake their nuclear deterrent, as Ukraine did, do so at considerable peril. After the Soviet Union invaded Afghanistan in 1979 the Senate refused to ratify Jimmy Carter's SALT II Treaty. Any serious response to Russia's aggression in Ukraine should include a formal and public U.S. demarche about Russian cheating on the INF treaty, while promising to withdraw from New Start if the cheating continues.

Nuclear arsenals aside, the timing of Mr. Obama's nuclear dismantling couldn't be worse as Mr. Putin contemplates his next moves in Ukraine and sizes up a possible Western response. Someone said recently that Mr. Putin plays chess while Mr. Obama plays checkers, but that's unfair to the noble game of checkers.