The White House now believes Syrian strongman Bashar Assad has used the poison gas sarin against his own people. Last August, President Barack Obama called the use of chemical weapons a "red line." He now faces a hard choice: Admit his red line was phony or intervene in a conflict he has sought to avoid.

The Syrian crisis is not just about sarin. Assad has killed more than 70,000 people since the uprising started two years ago. Obama intervened in Libya in 2011 for far less than that. And unlike Libya, Syria is strategically important: It is the conduit for Iranian aid to the Hezbollah terrorists in Lebanon.

Sarin is nasty stuff. It can be absorbed through the skin, and it kills by suffocation in under a minute. It was invented by Germany before World War II. Even Hitler decided it was better left unused, though his admirer Saddam Hussein did use it against the Kurdish people in 1988.

Assad is a murderer many times over, though his sarin has likely claimed only a few victims so far. Still, its use is a milestone. Since World War II, we view the use of weapons of mass destruction with far more horror than we do other kinds of violence. Obama reinforced this with his "red line" statement. If he didn't mean it, he shouldn't have said it.

The president's words push the United States toward intervention in Syria. Yet his administration wants to do less intervening, not more. It has wound down Iraq and is losing Afghanistan on the installment plan. The intervention in Libya came only because of pressure from Britain and France.

In Syria, the United States would have no meaningful support from our allies and would face bitter diplomatic opposition from Russia. While Sens. John McCain (R-Ariz.), Marco Rubio (R-Fla.) and Robert Casey (D-Pa.) have called for greater U.S. involvement in Syria, broader support is lacking. Fantasies about covert U.S. missions to round up Assad's chemical weapons belong to Hollywood. Above all, while an intervention might topple Assad, it won't save Syria from civil war after he goes.

The natural response of anyone facing a hard choice is to deny it exists. So the White House is now in denial mode. It calls for "a comprehensive United Nations investigation" in Syria, yet Assad has stonewalled the existing UN commission. It demands "credible and corroborated facts" about Assad's gas use, but carefully gathering all the facts in a war-torn dictatorship is impossible.

Obama has kicked the Syrian can down the road for two years. By choosing this approach, the president has trapped himself: He can back a rebellion that is increasingly dominated by radical Islamists, or he can ignore the growing evidence of Assad's violation of the White House's own red line.

After withdrawing from Iraq and leaving Qatar to manage the Syrian rebellion, the United States has little respect left in the region. Ignoring Assad would further reduce the credibility of other U.S. red lines that are vital to our national security, most notably our stand against the Iranian nuclear weapons program. The Iranians must be dangerously close to realizing that the United States will respond to nothing less than a nuclear test.

But it's not just Obama who faces a dilemma. We are all in this together. In 2002, President George W. Bush rightly said that the "gravest danger to freedom lies at the perilous crossroads of radicalism and technology." When the Syrian war ends, the thousands of Islamist radicals fighting there will move on.

Boston was just brought to a standstill by two suspected radicalized losers with homemade bombs who hailed from Chechnya, where Russia waged a war that had nothing to do with us. We are no more responsible for Syria than we were for Chechnya, but that won't stop the radicals from targeting us. And when they leave Syria, they may be carrying something far more dangerous than pressure cookers.

-Ted R. Bromund is a senior research fellow in The Heritage Foundation's Thatcher Center for Freedom.