By JON KYLMr. Kyl, a Republican, is a U.S. senator from Arizona.

Within hours of reassuming the Russian presidency this month, Vladimir Putin demanded that the United States provide “firm guarantees” that a U.S.-led missile-defense system in Europe won’t be aimed against Russia. President Barack Obama can’t offer any “legally binding” guarantee, because when he sought ratification of the New Start Treaty in 2010 he promised to accept no limits on U.S. missile defenses. But he can offer Mr. Putin a political assurance—what he might have been signaling when a “hot mic” recently caught him telling Russian leaders that he’d be “more flexible” after the November elections.

Offering any such assurances would be a serious mistake. American missile defenses aren’t targeted at Russia—they’re meant to defend against strikes by Iran and North Korea (and accidental or rogue launches, whatever their origin). But that’s not the point.

The right to self-defense is not one for which we must negotiate; it’s certainly not something for which Russia would negotiate. Yet, in an increasingly dangerous world, President Obama might be putting this most fundamental right on the table, presumably as a quid pro quo for the yet-to-be-realized benefits of “reset” with Russia.

We’ve been down this road before. In 1972, U.S. officials agreed to the Anti-Ballistic Missile (ABM) Treaty, effectively signing away America’s capacity to defend against nuclear attack. That decision was driven by the theory that missile defenses would spur an arms race, leading the Soviets to build more missiles to overcome our defenses. But the Soviets built those missiles anyway, increasing their number of strategic nuclear warheads to more than 11,500 in 1989 from about 2,500 in 1972. (U.S. warheads grew to about 13,500.)

Giving up the right to self-defense did nothing to improve security on either side, so in 2002 the U.S. wisely withdrew from the treaty in order to develop an effective U.S. ballistic missile-defense system. President George W. Bush provided no written assurances to Russia that our systems would not be directed toward Moscow, but he offered to cooperate on missile defense, given an assumed collective interest in defending against emerging threats from nations such as Iran and North Korea.

Such cooperation has proven elusive because Russia is less interested in cooperating against Iran than in degrading our missile-defense capability. Going back to the Reykjavik Summit between Ronald Reagan and Mikhail Gorbachev in 1986, Russia has wanted to prevent the U.S. from developing effective missile defenses. That’s why Russia insisted that the New Start Treaty link offensive and defensive forces, and why it conditions cooperation with NATO on legally binding limits on the capabilities and numbers of our missile-defense systems.

NATO has said that its defensive systems aren’t directed against Russia, but the Russians are insisting on proof. In recent weeks, NATO officials have seemed to imply that they may provide the political assurances Mr. Putin is seeking. If they don’t, says the chief of the Russian general staff, “a decision to use destructive force pre-emptively will be taken if the situation worsens.”

But do the Russians provide us written assurances that their new mobile ballistic missile systems won’t be directed against Europe or the U.S.? Or that their nuclear bombers and submarine-launched ballistic missiles won’t target us? No, they do not.

Why then, must the United States and NATO justify missile-defense deployments that pose no offensive threat and are intended to defend chiefly against Iran but—depending on future developments—might be effective against Russian missiles as well? Well, says Mr. Putin, because otherwise he will withdraw from New Start and engage us in another arms race. That didn’t work out so well for the Soviet Union, and it would be foolish now.

President Obama has a responsibility to defend America against all threats. Assuring any nation that our missile-defense systems will be ineffective against their nuclear ballistic missiles is clearly at odds with that responsibility. Mr. Putin must be made to understand that a desire to cooperate is not the same thing as a willingness to trade away our fundamental right to self-defense, and that America will always retain the right to defend itself.

Mr. Kyl, a Republican, is a U.S. senator from Arizona.