Former top military adviser concerned president is 'running out of options' with Iran The former top military adviser is concerned things "could spin out of control."

The former top military adviser to both Presidents George W. Bush and Barack Obama said on Sunday that he's concerned escalating tensions with Iran "could spin out of control," stressing that the last thing the world needs is the United States going to war with the Middle Eastern country.

"My biggest concern is the president is running out of room, running out of options, and while the rhetoric goes back and forth on how close we came to hitting Iran just the other day, that this thing could spin out of control," retired Adm. Mike Mullen told "This Week" Co-Anchor Martha Raddatz. "The last thing in the world we need right now is a war with Iran."

Mullen, who boasts a lengthy military career and served as the chairman of the Joint Chiefs of Staff between October 2007 and September 2011, said politicians need to diplomatically attain their goal of preventing Iran from developing a nuclear weapon, and that Americans need to call their representatives to advocate against declaring war on Iran.

"I really would like to know that the American people who feel we should not go to war with Iran are pressing their congressmen, their senators and everybody in the public domain to make sure that no matter what happens with respect to where we are with Iran right now, that we do not go to war," he said on "This Week." I think the politicians need to figure out a way to achieve the objective, which is Iran without a nuclear weapon, without -- from my perspective -- without regime change, without going to war."

One of those politicians, House Armed Services Ranking Member Mac Thornberry told Raddatz that President Donald Trump "is clearly trying to navigate a fine line to show that you cannot attack Americans and American military equipment without having a response."

"He's very conscious of not getting on an escalatory ladder that leads to a military conflict that neither side wants," the Texas Republican said in an interview on "This Week."

However, 2020 presidential candidate Sen. Cory Booker, a Democrat who sits on the Senate Foreign Relations Committee, accused the president of "taking a belligerent course of escalation and provocation with Iran" since the beginning of his presidency.

"We pulled out of a anti-nuclear deal that gave us complete transparency into their nuclear program," Booker said on "This Week," referring to Trump's decision in May 2018 to pull out of the Joint Comprehensive Plan of Action, colloquially referred to as the Iran nuclear deal. "We literally isolated ourselves from our allies and set us out on a very fragile limb towards conflict."

"The critical crisis we have is not just a drone being shot down, but now Iran has moved back to where it was before, which could be months from getting a nuclear weapon, which puts us again in that region on the brink of chaos," he added, also accusing the president of not having a strategy and making the U.S. "weaker."

In a series of tweets on Friday, Trump said that he called off a military strike on Iran with just 10 minutes to spare on Thursday night, explaining that the civilian casualties that would have occurred would not have been a "proportionate" response to the Iranians shooting down an unmanned U.S. drone late Wednesday.

Iran claimed the drone was flying in its airspace, but the U.S. government disputed that, saying it was flying in international airspace. Trump's reversal on the strike, which was first reported by The New York Times, was against the advice of Secretary of State Mike Pompeo and national security adviser John Bolton, sources told ABC News.

Mullen told Raddatz that a it's "very, very unusual," but not unprecedented "that a strike would be called off so close to its execution."

However, leaving for Camp David on Saturday, Trump told reporters "we hadn’t made a decision to go forward" with striking Iran when the retaliatory response was called off, later tweeting, "I never called the strike against Iran 'BACK,' as people are incorrectly reporting."

The president reiterated to reporters that his reason for stopping the strike was because he didn't want to kill 150 Iranians "unless it's absolutely necessary."

The United States did respond with a cyber strike against the country, The Washington Post reported on Saturday and a source later confirmed to ABC News. Trump approved an offensive cyber strike against Iranian computer systems used to control rocket and missile launches. The cyber attack was launched Thursday and was in the works for weeks if not months.

Thornberry told Raddatz on "This Week" that when Trump met with Democratic and Republican congressional leaders Thursday, himself included, "everyone agreed that shooting down an unarmed American aircraft deserved a response," and that all agreed "it should be on the lower end of the range of possibilities."

However, he added, "This story is not over," saying the Iranian's response to the cyber strike matters.

"Obviously the president has a whole range of additional responses that he could employ," Thornberry said. "There are a number of other military and probably other actions that could be taken if the Iranians decide that they want to continue this aggressive provocative sort of behavior."

The top Republican on the Armed Services Committee credited the president for seeking bipartisan input on how the United States should respond, but Booker was critical of the fact that a military response was considered at all, saying on "This Week" that Trump can't take military action against Iran without congressional approval.

"The Constitution speaks very clearly on this that he needs to come to Congress before he engages in military action that again could have us tumbling towards chaos and war in that region," he said.

"This situation is getting more and more tense, not less. We have a president that seems to be doing this like a reality TV show when trying to build more drama and trying to make a foreign policy by tweet," Booker added. "We have to, as a nation, work in coordination with our allies to denuclearize Iran, and to bring stability and peace back to that region."

Mullen said that if Iran does begin enriching uranium again and looks to be on its way to developing a nuclear weapon, it may lead to Israel attacking the country.

The former Joint Chiefs chairman also warned, "Iran with a nuclear weapon would start to proliferate nuclear weapons in the Middle East, which is incredibly dangerous. Other countries would then probably generate that kind of capability. And the Middle East has got a lot of problems and we don't need more nukes."

Booker echoed that warning, saying, "We're closer to a nuclear weapon, which could trigger proliferation around the region. It could trigger a military conflict and have us tumbling back into a Middle East war that will cost American lives and trillions of American dollars."

Thornberry said on "This Week" that Trump is "giving the Iranians every opportunity to back out of this cycle of increasing violence."

He said Trump shouldn't be criticized for giving the country the opportunity, but added that there's a limit to that.

"If Iran goes back to mining tankers, the sorts of things they've been doing here lately, then we have a whole range of military and other responses which we can employ, and I think the president will look to do that," he said.

But Mullen cautioned that if Iran gets a nuclear weapon, "attacking militarily is a very, very difficult task to actually make it happen."