Washington (CNN) President Donald Trump is adopting an increasingly cavalier attitude to the consequences of a war with Iran amid fears the current confrontation could spill over into a military conflict.

Trump canceled planned US strikes on Iranian targets last week, reasoning that a possible death toll of up to 150 was not proportional for the shooting down of a US drone.

But since then, he's spoken in breezy terms about the reverberations of a military showdown that both sides say they don't want but is a risk given the level of hair trigger tensions.

"It wouldn't last very long," Trump said on Fox Business Network on Wednesday.

A day earlier, bolstering his rhetoric amid worsening tensions with Tehran, the President said he didn't need an "exit strategy" in the event that any conflict breaks out.

Trump's comments raise the question of whether he has properly thought through the escalatory possibilities of a clash that could send inflammatory shockwaves throughout the Middle East.

A "shock and awe" debut to any war engineered by the US's massive edge in fire power would give the US an early advantage. But Iran, with its proxies throughout the region and beyond has the capacity to wage operations that could badly hurt the US.

Its Shia militia allies in Iraq could find soft targets among US forces. Its proxy Hezbollah in Lebanon could send missiles towards Israel. Iranian backed groups in Syria and Yemen could go after US targets. Saudi Arabia, the top US ally in the region is vulnerable. And Iran also has allies that could strike Americans in Afghanistan.

Tehran could call upon such asymmetric methods even if the US hopes to contain a war with limited strikes.

So absent an elimination of Iran's revolutionary political and military leadership it would not be an easy conflict to win, and even then, it's unlikely that any war would be short.

Reviving fire and fury rhetoric

There's always the possibility that Trump's offhand comments are part of a strategy designed to intimidate Iran to enter talks with his administration. His "fire and fury" approach with North Korea was similar — but the Iran situation is very different, given there is no sign Tehran wants the dialogue and validation from Washington that Pyongyang sought for years.

Trump's warnings will also kindle fresh debate over whether the commander-in-chief has sufficiently defined the political and strategic goals of any US military mission against Iran.

His comments last week that he did not learn the likely casualty toll of a US attack until 10 minutes before it was due to start did not ring true to many observers given the detail of pre-action briefings offered by the Pentagon to presidents as they weigh up their options in times of crisis.

Still, Trump told FBN on Wednesday that any such engagement would not be prolonged owing to conventional US military superiority.

"Well I hope we don't (go to war with Iran) but we're in a very strong position if something should happen," Trump said.

"It would not last very long."

'I don't need exit strategies'

The President made avoiding Middle East quagmires a centerpiece of his 2016 election strategy with Americans weary of years of military engagement following the September 11 attacks in 2001. His decision to step back from US strikes last week won praise even from people who usually criticize him. And Trump is far less hawkish towards the Islamic Republic than some leading members of his national security team.

With that in mind, he implied Wednesday that any conflict with Iran would use US air, missile and naval assets and would not result in the kind of large scale invasion seen in Iraq.

"I'm not talking boots on the ground, I'm not saying we're going to send a million soldiers, I'm just saying if something would happen, it wouldn't last very long," he said.

On Tuesday, Trump warned Iran that an attack on "anything American" would be met with "great and overwhelming force" that in some areas could mean "obliteration."

And asked whether he had come up with an exit strategy for any war with Iran, Trump replied: "You're not going to need an exit strategy. I don't need exit strategies."

If there is one lesson from America's two bloody past decades it is that wars in the Middle East and Eurasia are far easier to start than to finish and that talk of swift conflicts is hubristic.

Days after the September 11 attacks, then President George W. Bush warned US enemies that the war on terror would end "in a way and at an hour of our choosing."

Eighteen years on and despite US successes against Al-Qaeda and ISIS, the US still has not worked out how to get all its troops home from Afghanistan, which is still dangerous and unstable.

Before the Iraq war, some US officials, including then Defense Secretary Donald Rumsfeld, predicted that it would not last very long and be over in a matter of weeks or months. In the event, a swift US toppling of Saddam Hussein sparked an insurgency that killed thousands of Americans.

Trump's comments about a hypothetical war with Iran recall his comments about another area of non-military conflict.

"Trade wars are good and easy to win," Trump tweeted in March 2018. Subsequent events, including in the tariff war with China, have proved that comment was overly optimistic.