Would we accept the humiliation—or escalate? What if Syrian air defenses start bringing down U.S. planes? What would we do if Syria’s Hezbollah allies start taking Americans hostage in Lebanon?

Ronald Reagan sent the Marines into Lebanon in 1983. His intervention in that civil war resulted in our embassy being blown up and 241 Marines massacred in the bombing of the Beirut barracks. Reagan regarded it as the worst mistake of his presidency. Are we going to repeat it because Bashar has failed to live up to our expectations?

Consider the forces lining up on each side in what looks like a Syrian civil war and dress rehearsal for a regional sectarian war.

Against Assad’s regime are the United States, the Muslim Brotherhood, al-Qaida, the Turks and Saudis and Sunni states of the Persian Gulf.

On Assad’s side are his 300,000-strong army, the Alawite Shia in Syria, Druze, Christians and Kurds, all of whom fear a victory of the Brotherhood, and Russia, Iran and Hezbollah.

The question for our bellicose interventionists is this:

How much treasure should be expended, how much American blood shed so the Muslim Brotherhood can depose the Assad dynasty, take power and establish an Islamist state in Syria?

“Tell me how this thing ends,” said Gen. David Petraeus at the onset of our misbegotten Iraq War. If we begin providing weapons to those seeking the overthrow of Assad, as the Post urges, it will be a fateful step for this republic.

We will be morally responsible for the inevitable rise in dead and wounded from the war we will have fueled. We will have committed our prestige to Assad’s downfall. As long as he survives, it will be seen as a U.S. defeat and humiliation.

And once the U.S. casualties come, the cry of the war party will come—for victory over Assad, Hezbollah, Iran, Russia! We will be on our way into another bloody debacle in a region where there is no vital U.S. interest but perhaps oil, which these folks have to sell to survive.

Before the religious and ethnic conflicts of Europe were sorted out, it took centuries of bloodletting, and our fathers instructed us to stay out of these quarrels that were none of our business.

Syria in 2012 is even less our business.