Trump is breaking all of Obama's rules in Syria — and it seems to be working

Trump, a political neophyte then inside his first 100 days in office, attacked an ally of Russia and Iran after intelligence services concluded that Assad's forces had used chemical weapons on Syrian civilians, many of them children.

But Syria never fired back. Neither did Russia. And so far, Iran hasn't either. The salvo of 59 cruise missiles that took out a handful of Assad's warplanes went virtually unpunished.

The incident typifies the difference in Trump's and President Barack Obama's Syria policy, in which Trump seems to have successfully called Iran's bluff.

Obama was pressed by a similar situation in 2013, after evidence surfaced that Assad violated Obama's "red line" by using chemical weapons. Instead of following through on his threat to hit Assad in response, Obama agreed to let Russia step in and deal with the chemical-weapons stockpile.

Toward the end of Obama's term, it became clear why he had shied away from striking Assad: He was focused on the Iran nuclear deal.

"When the president announced his plans to attack [the Assad regime] and then pulled back, it was exactly the period in time when American negotiators were meeting with Iranian negotiators secretly in Oman to get the nuclear agreement," Wall Street Journal reporter Jay Solomon told MSNBC last year.

"US and Iranian officials have both told me that they were basically communicating that if the US starts hitting President Assad's forces, Iran's closest Arab ally ... these talks cannot conclude," Solomon continued.

But Trump has patently different ideas about Iran. He vocally opposed the Iran deal and campaigned on tearing it up. While Trump hasn't followed through, his administration has moved to put additional sanctions on Tehran, as the deal has freed up over $100 billion of Iran's funds.

And importantly, Trump has shown he'll hit Assad if needed and even hand over power to battlefield commanders to hit Iranian-backed forces if they threaten US troops.

Obama's refusal to enforce his red line or punish Assad militarily for a host of war crimes Assad has been accused of committing under his watch "was never about fear of World War III," said Jonathan Schanzer, an expert on the Middle East from the Foundation for Defense of Democracies.

"The fear for Obama was upsetting the nuclear deal. That was what they were protecting. It wasn't about sparking some wider confrontation," said Schanzer, alluding to Russia's 2015 entrance into the conflict on Assad's behalf.

So while Obama walked on eggshells with Iran to preserve his deal, apparently believing Iran would exit if he acted against it, Trump has had the benefit of entering office post-deal.

Every review of Iran's nuclear facilities by the International Atomic Energy Agency since Trump took office has shown that Iran is complying with the deal's terms. To outside observers, Iran appears in line with the letter of the deal, even after the US's April 7 strike on Assad's airfield.

But the tension between the US and Iran hasn't resolved — it has shifted. Nick Heras, an expert on Syria with the Center for a New American Security, told Business Insider that Iran's attention had settled on eastern Syria, where a US-led coalition is getting ready to dislodge ISIS.

"In eastern Syria, Iran is trying to box the US out," Heras said. "The Iranians don't want the US to open up shop in eastern Syria. Iranians have sent columns of militias to try to force out the US in eastern Syria. The Iranians assess that there's a threat that the Trump administration would build up a presence to try to stabilize eastern Syria."

Iran has not taken kindly to the idea of increased US influence or presence in Syria. Since May, the US-led coalition has responded three times to what it perceived as attempts by pro-Assad, Iranian-backed forces to attack it. And each time, US air power has devastated Iran's proxies.

"I believe that Trump's instincts on the Middle East are not bad," Schanzer said. "He understands that he needs to project strength to these actors, and he is. That's giving us more leverage with actors that in the past Obama was fearful of challenging, and that's positive."

But while the US is no longer being coerced into walking an Iranian-approved path in Syria, clashes with Iran could put the about 500 US troops in Syria at risk, as the US closes in on ISIS's final strongholds and the fight for the future of Syria shapes up.