As a general, Mattis urged action against Iran. As a defense secretary, he may be a voice of caution.

Retired U.S. Marine Corps General James Mattis has been chosen to be secretary of defense by President-elect Donald Trump

The Washington Post, January 8, 2017 - The Iranian-supplied rockets were raining down on Gen. James N. Mattis’s troops throughout the spring and summer of 2011 with greater and greater intensity.
Six American soldiers were killed by a volley in eastern Baghdad in early June. A few weeks later, three more Americans died in a similar strike, driving the monthly death toll to 15. It was the worst month for U.S. troops in Iraq in more than two years, and Iran’s proxies were vowing more rockets and more bloodshed.
Mattis, the top American commander in the Middle East, was determined to send a clear message to Tehran to stop it. His proposal, crafted with the support of the ambassador and the senior American commander in Iraq, was to hit back inside Iran, said current and former senior U.S. officials, who took part in the debate.

Iran nuclear deal is in the crosshairs and may not survive a Trump administration

The Washington Post, Jan. 11, 2017 - The Iran nuclear deal was written with several “sunset” provisions setting expiration dates, some of them 15 years into the future, when restrictions on the Islamic Republic’s nuclear program would lift.
Then Donald Trump was elected president and a sunset on the deal itself became possible. During the campaign, Trump regularly denigrated the Iran agreement. He vowed, at turns, to walk away from the 2015 accord, or renegotiate it, or enforce it so rigorously that it might collapse on its own.

Trump is expected to take a more confrontational approach with Iran, showing no tolerance for even small breaches of what was agreed upon. The strategy seems designed to increase pressure on Iran, stopping what critics consider backsliding or cheating, but also to compel it to moderate its actions elsewhere in the region.

Trump will take office a year and a half after the agreement was reached in Vienna, limiting Iran’s ability to amass enough material to build nuclear weapons without the world receiving advance warning. Although some aspects will remain in force for 10 or 15 years, Secretary of State John F. Kerry said Iran would be required to grant inspectors from the International Atomic Energy Agency access to its nuclear facilities “forever.”

The new administration can renegotiate the Iran nuclear deal from a position of strength.

By; Michael Makovsky
The Wall Street Journal, Jan. 3, 2017 As the bipartisan opponents of President Obama’s Iran nuclear agreement prepare to address its many shortcomings, they should beware of unwittingly repeating some of his mistakes.
Instead of relying on more sanctions to dismantle or renegotiate the deal, the most urgent need is restoring U.S. credibility and resolve in opposing Iranian aggression and reshaping the Middle East.
Two fundamental misjudgments led to the disastrous nuclear agreement. First, Mr. Obama eschewed credible military threats and relied on congressionally generated economic sanctions to pressure Iran to negotiate. Second, he focused only on Iran’s nuclear program, ignoring its malignant regional misconduct. Free of pressure and scrutiny, Tehran shaped the agreement’s terms and expanded its aggression and influence.
The current policy debate has ignored these mistakes. Instead, it is focused on using sanctions to enforce and improve the Joint Comprehensive Plan of Action with Iran. This narrow approach is counterproductive. The agreement front-loaded Iran’s economic benefits. But it only mothballed elements of its nuclear program; it did not eradicate it.
The U.S. will need years to rebuild a robust international sanctions regime