As promised, Iran has delivered its response to the U.N. ultimatum that it resume negotiations over its nuclear-energy program. And, as expected, the response was sufficiently ambiguous to offer something to everyone. If Tehran is serious about talks and is truly seeking a negotiated solution to stave off a crisis, then other nations must make every effort to seize the opportunity. If Iran is merely stalling for time to present the world with a nuclear fait accompli, then the world -- and the United Nations in particular -- must be ready to force Tehran to meet its international obligations.

Four years ago, revelations about clandestine nuclear facilities hardened suspicions about the purpose of Iran's nuclear program. The slow drip of evidence and the belated acknowledgment that Tehran had not been totally forthcoming with the International Atomic Energy Agency, the U.N.'s nuclear watchdog, compelled the world body to take a harder line when dealing with Iran. The announcement that the country had begun enriching uranium, as well as its increasingly belligerent rhetoric, compounded fears that Iran sought nuclear-weapons capability.

Earlier this summer, after several years of on-again, off-again negotiations, the five permanent members of the U.N. Security Council and Germany (which had helped lead European Union discussions with Tehran) offered Iran a package of political, economic and security incentives to halt its nuclear program. Iran wanted time -- two months -- to study the proposal and prepare a response.