Once we accept enrichment in Iran, it will become impossible to deny the same arrangement to friendly governments in the region, let alone unfriendly ones like Syria. The result will be the proliferation of dangerous nuclear technologies that we have been seeking to avoid.

The risks will be even greater if we agree to convert Natanz into an international enrichment center. International partners will not invest in a primitive enrichment operation that relies on old and unproven technologies. They will insist on state of the art enrichment equipment, Western management and access to export markets — the absence of which has hindered Iran’s enrichment progress up to now. By contrast, so long as Iran’s nuclear enrichment program remains illegitimate and subject to international censure, it cannot serve as an attractive model for other countries.

For these reasons, the United States cannot be more eager than Tehran to reach a deal, and Mr. Obama must persuade Iran that he can afford to see negotiations fail.