President Obama's cautious approach to foreign policy and his reluctance to entangle the United States in overseas' conflict has largely served his administration well. Recognizing that the American public is weary of U.S. military interventions after Iraq and Afghanistan, President Obama has been guarded in exerting American power. It now appears that concerning Syria, at least, his administration was too cautious, contributing to bad results and leaving limited options.

Two years ago this month the president declared that "the time has come for (Syrian) President Assad to step aside." Who could object? President Assad appeared set to be the next despot to fall as a yearning for democracy and the rule of law swept the region. It seemed a clear signal to rebel forces that they had a friend in the United States.

Unanswered, however, were who and what kind of government would replace Mr. Assad and how would his overthrow be achieved? The internal Syrian conflict was complicated by religious and tribal alliances that spread beyond that pivotal nation's borders. Mr. Assad's government would not go quietly. The death toll from the civil war has left more than 100,000 dead and displaced an estimated one-quarter of Syria's 23 million people.

Early in the conflict the United States had the option of backing the moderate Free Syrian Army and giving it cover by using U.S. air power and cruise missile strikes to weaken Mr. Assad's army. But the interest of the U.S. public was not engaged and intervention in another war would not have proved popular on the home front. The administration opted to monitor the situation.

One year ago, President Obama issued his "red line," stating that the movement or use of chemical weapons by the Assad regime "would change my calculus" concerning U.S. involvement.

When this past June came word of a small-scale use of chemical weapons, the White House announced it would provide military aid and some arms to the Syrian opposition. But in the intervening time since the conflict began, the calculus had become more complicated. Islamist jihadists, including groups associated with al-Qaida, had gained the upper hand among the rebel forces. If Assad did go, it appeared probable these radical groups would take control or, at the very least, Syria would remain divided amidst competing factions. With confusion over who could be safely armed among rebel forces, the United States has provided little of the promised help.

The Shiite nation of Iran and its proxy, Hezbollah, are backing the Assad regime. Sunni-dominant Saudi Arabia has hedged its bets, supporting the moderate forces preferred by the United States, but also sending its wealth to radical jihadist groups because they appear to have the greater chance to prevail. The threat of Syrian chaos spreading to neighboring nations is serious, with dire implications for interrupting the flow of Mideast oil and providing a training ground and haven for jihadists.

Now the world confronts a larger chemical attack on hundreds of Syrian civilians. Secretary of State John Kerry said Monday that the Assad government's responsibility for the "cowardly crime" is undeniable.

If future U.S. "red lines" are to have any credibility, a military response to strike at Assad's fighting capacity appears unavoidable, perhaps through cruise missile attacks to destroy missile launch sites and damage Syria's air force. Aside from foreign policy, the United States has a moral obligation to address the atrocity.

Preferably, China and Russia could be persuaded to support a military response. Unfortunately, given those nations cynical policy approach to the Syrian conflict, their support appears unlikely. Instead, the White House will probably have to seek a coalition of willing allies.

And with those allies the United States must figure out a way, albeit belatedly, to provide support to the moderate rebel forces and improve their chances of prevailing in a post-Assad Syria. While such an effort may not prove determinate, evidence grows it is far better than the administration's approach up to now, which has only emboldened Mr. Assad and increased the uncertainty about what would follow his fall.