The Washington Post | Nicholas Miller: The Trump administration has an Oct. 15 deadline to certify to Congress that Iran is complying with the terms of the 2015�Joint Comprehensive Plan of Action, commonly known as the Iran nuclear deal.

Since January, the Trump administration has issued this certification every three months, and the International Atomic Energy Agency has repeatedly confirmed�that Iran is in compliance�� but the October certification remains unclear.

In a Sept. 19 address to the United Nations, Trump called the deal �an embarrassment to the United States� and said, �I don�t think you�ve heard the last of it.� In recent months, the administration claimed that Iranian missile tests and other regional activities��violate the spirit��of the deal. On Saturday, Trump responded to an Iranian missile test by declaring,��Not much of an agreement we have!�

What happens next will send a far broader signal about the U.S. commitment to nonproliferation�

Although a decision to �decertify� Iran would not immediately blow up the deal,�it could lay the groundwork�for Congress to reimpose sanctions on Iran. This, in turn, might lead Iran to exit the agreement and ramp up its nuclear program to pre-2015 levels, raising the risk of proliferation or preventive war.

Trump may be using the threat as�leverage to renegotiate the deal, but he faces a rocky road given�Iranian opposition�and the reluctance of many of the other P5+1 partners involved in brokering the deal: China, France, Germany, Russia and Britain.

Trump�s decision is important not only because of its implications for Iran and the wider Middle East; the decision is also crucial because of what it will communicate about the broader U.S. commitment to nonproliferation.

U.S. nonproliferation efforts have achieved notable success

For decades, preventing the spread of nuclear weapons has been a�top U.S. priority. As I argue in�a forthcoming book, U.S. policies help explain why only nine countries have nuclear weapons today � in contrast with the�much higher numbers�forecast in the early years of the nuclear age.

Historically, Washington�s effort to limit the spread of nuclear weapons has rested on four key pillars, but each is showing signs of crumbling:

1) Credibly opposing proliferation

In the late 1960s, the United States�worked with the Soviet Union�on the�Non-Proliferation Treaty, which laid the foundation of the nonproliferation regime.�Starting in the 1970s, the United States has threatened and imposed sanctions against friends and foes alike that have sought nuclear weapons.

Now�mixed signals come out of Washington. During the 2016 campaign,�Trump said�it would be okay if Japan, South Korea or Saudi Arabia acquired nuclear weapons. In March, Secretary of State Rex Tillerson�refused to rule out�Japanese or South Korean proliferation.

2) Reassuring allies

A second essential element of U.S. nonproliferation policy is the extension of security guarantees and the U.S. nuclear umbrella to allied states. U.S. protection not only�reduces the odds�that allies feel the need to develop nuclear weapons, but it�also provides leverage�if an ally does begin seeking nuclear weapons.

Trump�s �America First��foreign policy�has raised significant doubts about U.S. alliance commitments, including�NATO,�South Korea�and�Japan. U.S. allies were already skittish for reasons unrelated to Trump � namely, Russia�s renewed belligerence and�North Korea�s rapid nuclear advances.�Today,�South Koreans,�Japanese�and even�Germans�have renewed debates about hosting nuclear weapons or developing their own nuclear arsenals.

3) Reducing the salience of nuclear weapons

Over the past few decades, the United States has significantly reduced the�size of its nuclear arsenal. President Barack Obama declared a goal of�moving to a world without nuclear weapons(but his administration also supported an expensive program to�modernize the U.S. nuclear arsenal).

The tone has markedly changed under the Trump administration. Shortly before taking office, Trump�welcomed an arms race and called for the United States�to �greatly strengthen and expand its nuclear capability.� In recent months, Trump has lobbed a number of grisly nuclear threats, warning of��fire and fury��and promising to��totally destroy North Korea��in the event of a North Korean attack.

And the Trump administration reportedly is�considering developing�new �mini-nukes� with the aim of making nuclear weapons more usable in a conflict.

4) Providing a diplomatic exit to proliferators

U.S. nonproliferation policy also has succeeded when it offered adversaries a diplomatic off-ramp � by abandoning nuclear weapons programs, they can gain improved relations with the United States. In 2003, for example, the George W. Bush administration�agreed to lift sanctions on Libya�and drop a policy of regime change in exchange for Libya�s giving up its weapons of mass destruction programs. A similar principle informed the Iran deal, as the P5+1 lifted sanctions in exchange for limits on Tehran�s nuclear program.

This type of bargain is viable only when Washington can credibly assure its adversaries that it will uphold negotiated arrangements. The credibility of U.S. assurances�was already highly questionable�before Trump made the matter worse by threatening to scuttle the deal.

Over the past 15 years, the United States has launched an invasion of Iraq ostensibly for nonproliferation reasons ��even though it had already disarmed�� and supported the�overthrow of Moammar Gaddafi�even after he agreed to give up Libya�s weapons programs.

Here�s what this means for the Iran deal and U.S. nonproliferation policy

If Trump withdraws from the deal, it might permanently cement the perception that there is no durable diplomatic off-ramp for adversary proliferators.

Think of it this way: If the United States cannot be trusted to abide by a bargain and will sanction or invade your country even if you agree to limit your nuclear program, why would you agree to any limits? A viable nuclear deterrent is the one thing that might prevent a U.S. invasion, after all. This logic explains why�many analysts�warn that withdrawing from the deal would cripple any hopes of�achieving limits�on the�North Korean nuclear programdiplomatically.

Undermining the Iran deal would also strengthen the perception that Washington is not truly committed to opposing proliferation. A weakened or collapsed deal would increase the incentives for countries such as Saudi Arabia to seek their own nuclear weapons. And it would signal that the United States prioritizes preventing missile tests, hemming in Iranian support for proxy groups and achieving�regime change in Iran�over nonproliferation.

Given that several core pillars of U.S. nonproliferation policy are already showing signs of stress, the fate of the Iran deal may be even more important than it initially seems.

Nicholas Miller is an assistant professor of government at Dartmouth College. His book, �Stopping the Bomb: The Sources and Effectiveness of U.S. Nonproliferation Policy,� is forthcoming with Cornell University Press in 2018.