From arms control deals to this week’s Nuclear Security Summit in Washington, President Obama says his atomic diplomacy “will make America safer.” This sound bite deserves proper scrutiny, and fortunately the Senate can do that once the new Strategic Arms Reduction Treaty comes up for ratification.

Signed with some pomp last week in Prague, the pact with Russia makes modest reductions to the number of strategic warheads and delivery systems. Though those cuts are worth a close look, we’re much more concerned with the impact that new START will have on America’s ability to develop and deploy the best missile defenses available.

Starting with the Reagan-era Strategic Defense Initiative, the Kremlin has sought to tie America’s hands on missile defense. The Kremlin says that this is precisely what it has negotiated with START. The Administration says it didn’t. They can’t both be right.

Let’s go to the treaty text. The preamble notes “the interrelationship between strategic offensive arms and strategic defensive arms,” thus linking existing weapons and America’s missile defenses. Article V contains a binding clause that the U.S. or Russia “shall not convert and shall not use ICBM launchers and [submarine-launched ballistic missile] launchers for placement of missile defense interceptors therein.”

Speaking on March 30, Ellen Tauscher, the State Department’s chief treaty negotiator, said “there are no constraints to missile defense in the START treaty.” Someone must have pointed out the sections above, because on April 14 before the House Armed Services Committee Ms. Tauscher modified her take. “The Treaty does not contain any constraints on testing, development or deployment of current or planned U.S. missile defense programs,” she said. (Emphasis is ours.)

The distinction is more than academic. The Obama Administration may not currently plan to convert an ICBM silo into a missile defense site. But Mr. Obama won’t be in office beyond 2017, and a future President might want to. START wouldn’t allow it.

These limits may not add up to a new Anti-Ballistic Missile Treaty, which President Bush wisely withdrew from in 2002 in order to allow the U.S. to make full use of modern technology for its defenses. But START contains a political poison pill that does something akin to the ABM treaty by putting a high price on future missile defense development.

Article XIV lets any party withdraw if “extraordinary events . . . have jeopardized its supreme interests.” The Kremlin promptly clarified this to mean that START will survive “only if the [U.S.] refrains from developing its missile defense capabilities quantitatively or qualitatively.” In practice, this will mean that any new defense initiative will have to overcome a Russian threat to withdraw from START. Opponents of missile defenses in Congress and abroad will claim that any such move would be destabilizing and start a new “arms race.”

President Obama has said this pact is meant to be a first step toward “a world without nuclear weapons,” so he is unlikely to challenge this Russian interpretation. The Administration has cut spending for missile defenses, killing or moth-balling innovative programs to hit rockets in their early “boost” phase. In place of the Bush interceptors intended for Poland, Mr. Obama says the U.S. will deploy a ground-based Aegis Navy interceptor system in Europe by 2020. However, the Aegis system remains in development, and you can bet the Russians will raise objections eventually.

START’s reductions in nuclear weapons are tolerable, but restraints on missile defenses are not. Before it ratifies the pact, the Senate needs to insist that President Obama personally clarify the limits implied by the treaty text, and in clearer terms than Ms. Tauscher’s recent fudges. The Senate should also state clearly itself, in an addendum to the treaty if need be, that it is recognizing no limits on the ability of the U.S. to defend itself against missile attacks.