There are two major differences between President Barack Obama's red line debacle against Syria in 2013, and President Trump's last-minute decision late Thursday to cancel a series of strikes against Iran.

Those two factors are the cost to U.S. credibility and the betrayal of important allies.

Consider the divergence on the credibility factor. When, in August 2013, Bashar Assad's Syrian regime used chemical weapons against its own people, that action broke Obama's previously established red line against such action. Obama's threat had been delivered very publicly and it was understood that Assad would face U.S. military reprisals if he conducted any chemical attack.

But when Obama then failed to act (Congress' rejection of his authorization of force request is no excuse: Obama had clear constitutional authority to use limited military action in defense of American interests and credibility), America's word fell into question. Obama appeared to be the president who would blink. This weakness certainly reduced Obama's deterrent value for the rest of his presidency, especially in relation to China and Russia.

This isn't the case with Trump's action on Thursday.

For a start, Trump had not issued a clear red line to Iran prior to its attack on a U.S. drone, so the credibility-expectation factor was not present. But Trump also gave himself a carve-out to ensure the drone incident doesn't define his position on Iranian aggression.

During his White House meeting with Canadian Prime Minister Justin Trudeau, Trump observed that his response would have been much different had a manned aircraft been shot down rather than a unmanned drone. So if Iran now shoots down a manned aircraft or fires on a manned U.S. vessel, Trump will have to act to preserve U.S. deterrent credibility. We just aren't there yet.

Then there's the question of allies.

While it's a frequent refrain that the president dishonors our allies, Obama's red line debacle did so in the worst fashion. Alongside U.S. aircraft ready to hit Assad's regime in 2013 was the French military. When Obama abruptly changed course, he left French President Francois Hollande looking like an idiot. As a consequence, Obama left the French government, and thus the entirety of the European Union, with the perception that he is unreliable at critical moments. That is a perception, incidentally, that Obama did little to alter with his 2014 policy in Ukraine.

So where does this leave us?

I am sympathetic to Trump's Iran policy. U.S. sanctions pressure is driving Iran's economy into an abyss. That is greatly weakening the hardliner faction which controls nearly half of the economy. In turn, there is good reason to think Trump's action will force the more-moderate Iranian political bloc back to the table to negotiate an improved nuclear deal. If not, the regime risks implosion.

Until then, America can be confident in the good sense of avoiding use of military force unless a serious lethal attack occurs on Americans or our allies.

What about paying for the drone? We can take the $130 million from Iranian accounts seized by the U.S. government.