Editor's note: This is one in a series of stories and opinion pieces surrounding the Aspen Security Forum. which took place from July 17 to 20 in Aspen, Colorado. Security Clearance was a media sponsor of the event.

By CNN's Alison Harding

A former commander of U.S. Central Command said the United States needs to determine an endgame in Syria before it takes further military action in the beleaguered country.

In a panel moderated by CNN’s Wolf Blitzer, retired Gen. James Mattis told the Aspen Security Forum on Saturday that escalated involvement in Syria by the U.S. military would lead to “a full-throated, very, very serious war.”

When asked what if anything the U.S. should do to topple President Bashar al-Assad's regime, Mattis said the United States must first consult with regional powers to determine a framework under which to operate before engaging militarily.

“Then we need to be very clear about our military end state and political end state. Otherwise you'll invade a country, pull down a statue, and say, 'Now what do we do?'” Mattis said.

The Obama administration has been reluctant to enter another military engagement, but announced in June that it would provide military support to start rebel fighters after it determined al-Assad’s forces had used chemical weapons - a red line that President Barack Obama warned the regime not to cross. The U.S. has also been urged by some to establish a no-fly zone over Syria.

But Mattis cautioned that setting up a no-fly zone would be a complicated and costly endeavor that is not a pragmatic military solution in a conflict where most of the violence is occurring on the ground.

“We have no moral obligation to do the impossible and harm our children’s future because we think we just have to do something,” Mattis said. “The killing will go on on the ground because they’re not using aircraft to do most of the killing.”

The retired general also said the administration’s plan to supply arms to the rebels is not without risk, as the weapons could get into the wrong hands. But that risk can be mitigated by thorough training, employing the secret services of surrounding countries and using U.S. special ops to monitor the situation.

“There's a way to do it, but it’s a commitment, not a donation,” Mattis said. “This is significant for a country that is once more going to find itself at odds in the midst of a very, very confusing situation on the ground in the Middle East.”

Syrian rebels have faced a series of tactical setbacks in recent weeks as al-Assad’s forces took back key strongholds - a development Mattis said was made possible by the support al-Assad receives from outside powers who have an interest in keeping him in power - particularly Iran.

“Bashar al-Assad has gotten the full support out of Tehran and out of Lebanese Hezbollah,” Mattis said.

“Absent that orchestrated support ... I think Assad would have found himself overrun by the gathering momentum against him,” Mattis said.

Mattis said if al-Assad falls, it would be the “biggest strategic setback in 25 years” for Iran, which counts Syria as one of its few remaining allies as the international community continues to impose harsh economic and diplomatic sanctions on Iran over its highly secretive uranium-enrichment program.

When asked how close the country is to a nuclear weapon, Mattis predicted they could have one within a year if they choose to.

“I don't believe (Iranian Supreme Leader Ayatollah Ali Khamenei) has made the decision. If he does, I am not completely confident that we would know immediately. I don't think we'd know right away,” Mattis said.

He offered reserved optimism that Iran’s newly-elected president, Hassan Rouhani, could help his country walk back the nuclear weapons program, but predicted the new leader would not get much support in the current political climate in Tehran. Mattis said nonetheless, the United States should try to work with the new leader, but should do so with “modest expectations.”

One thing Mattis said he knows for certain is that should Iran get a nuclear weapon, Israel would launch an airstrike on Iran’s nuclear facilities. But he cautioned that even with potential assistance from the U.S., it would be nearly impossible to destroy Iran’s massive setup.

Mattis said ultimately, the only real resolution to Iran’s nuclear program will be a diplomatic one.

“The military can buy our diplomats some time, but it cannot solve this problem,” Mattis said.

Throughout his talk, Mattis championed the role diplomacy plays in military engagements throughout the world, saying “not a week went by, not a day went by when I didn’t talk to the 20-odd ambassadors in my region.”

Mattis said he often lobbied the House and Senate Foreign Relations Committees to increase funding of diplomatic efforts, saying he would tell them, “if you don’t vote more money for the foreign relations budgets to help our ambassadors to reach out and make an impact, please vote more money for ammunition because I’m going to need it.”

In the wide-ranging interview, Blitzer also asked Mattis if the war in Iraq was worth it, given the tremendous American sacrifice over the past 10 years and the Iraqi government's reluctance to do more to stop Iranian weapons shipments to Syria over Iraqi airspace.

"If Iraq, sitting in the geo-strategic center of the Middle East, continues to mature in a democratic way, then I would say yes," he said.

–CNN's Shirley Henry contributed to this report.