According to statements from U.S. and Russian officials, a new strategic arms agreement to replace the one that expired last December is near completion. It has proved more difficult than expected, and the agreement will face intense scrutiny from the U.S. Senate.

A major problem has been the connection between offensive and defensive arms, a linkage that recalls the first strategic arms agreement, SALT I, in 1972. Then it was the Soviet Union that had an operational strategic defense system around Moscow, while the U.S. only had an incipient program. U.S. officials insisted there would be no agreement unless limits on defensive arms were included. After initially resisting, the Soviets agreed, and the final arrangement limited both offensive and defensive arms.

Now the tables are reversed. The Soviet Union is no more, and Russian capabilities to engage in an arms race are limited. In the meantime, the U.S. has made progress in developing missile defenses. The emergence of new threats, notably Iran, has led to increased interest in such defenses, both to defend the United States and to reassure allies.

An arms control agreement has both technical and political underpinnings, and it is a challenge to reconcile both imperatives. When President Ronald Reagan announced the strategic defense initiative, more commonly known as “star wars,” he said he wanted to make nuclear weapons “impotent and obsolete.” To reassure the Soviets that he did not want to gain a military advantage, he offered to share the technology. But these assurances were not taken seriously. Although President Reagan was able to achieve a treaty abolishing intermediate nuclear forces, a strategic arms control agreement eluded him.

The shift in the balance since then serves as a further complication to reaching an agreement. In 2006, Foreign Affairs, a leading publication in its field, published an article claiming the U.S. could disarm Russian nuclear capabilities in a first strike. “Russia’s leaders can no longer count on a survivable nuclear deterrent,” wrote Keir A. Lieber and Daryl G. Press. “And unless they reverse course rapidly, Russia’s vulnerability will only increase over time.”

To be sure, such an analysis is controversial, but those responsible for the defense of their country are inclined to take the worst-case scenario seriously. Arms control agreements work best when they provide assurance that allows leaders to dismiss the worst cases, allowing for further disarmament and reduction in tensions.

An example is the Rush-Bagot agreement of 1817. Concerned by the possibility of an arms race on the U.S. border with Canada, the British and American governments agreed to limit their military deployments. The U.S. border with Canada subsequently became the longest undefended border in the world.

When the Cold War ended, there were hopes of greater collaboration between the United States and Russia. And although there have been tremendous disappointments, there has been some cooperation. For example, Russia is allowing the United States to use its territory to supply American armed forces in Afghanistan.

Will such cooperation continue and expand if Moscow becomes convinced the U.S. is trying to attain nuclear primacy? That is the political question underlying the strategic arms agreement.

Although Russia does not have the power of the Soviet Union, the U.S. also has seen its position weaken. In particular, the financial crisis means budgets will have to be cut, and it is doubtful defense will be able to escape the knife. Strategic weapons, both offensive and defensive, will have to be weighed against the need for supporting conventional forces.

Accordingly, the U.S. should be careful about not exaggerating its capabilities. When the original SALT agreement was negotiated, the U.S. had an advantage in MIRV (multiple independently-targetable re-entry vehicle) technology. But the Soviet Union soon was able to master the technology, and since its missiles were bigger, the threat it posed was formidable. The advantage the U.S. thought it had turned out to be not so advantageous, after all.

History does not repeat itself exactly, but it has lessons that should not be ignored. The INF treaty that President Reagan reached with Soviet President Mikhail Gorbachev had a symbolic political impact that contributed significantly to the end of the Cold War. We should not ignore those political dimensions as we assess the treaty that our negotiators are now completing.