Carolyn Kaster, Associated Press
People rallying for U.S. military action in Syria shout to opponents in front of the White House on Saturday, Aug. 31, 2013, in Washington. A sign at right depicts Syrian President Bashar Assad in the likeness of Adolf Hitler.

If one thing has become clear in recent years, it is that House Republicans won't necessarily follow in lock-step with their speaker, John Boehner. So when Boehner and House Majority Leader Eric Cantor both announced this week they support President Obama's call for a limited military strike against Syria, that was good news for the president, but not necessarily a signal that victory was imminent.

And House Republicans aren't Obama's only worry. Many members of his own party are equally skeptical about the need for a military strike in retaliation for what the administration says is clear evidence the regime of Bashar al-Assad launched a deadly chemical attack on Syrian civilians.

All of which makes Obama's surprise decision Saturday to seek congressional approval remarkable. The president should be commended for doing so in an age when presidents often presume that military incursions don't require approval from the people's representatives, and especially when the outcome of such a request is so uncertain. Now it is up to him to make a convincing case. That process got off to a good start Tuesday when Secretary of State John Kerry and Secretary of Defense Chuck Hagel appeared before the Senate Foreign Relations Committee.

Opponents of military action rely on several arguments. Among them: The most recent chemical attack was not the first attributed to Assad, and yet the United States chose not to respond to the previous ones. Other dictators, most notably Saddam Hussein, used chemical weapons against their own citizens without prompting a response from the U.S. Some argue it's illogical to draw the line on chemical weapons while allowing dictators to kill people with conventional weapons without a similar response. People killed by bullets are just as dead as those killed by sarin gas. Also, they wonder why the administration has not armed Syrian rebels as it said it would and why Obama is insisting military strikes would not be intended to topple Assad's regime. Finally, they say the administration has yet to define a clear national interest in Syria's internal conflict.

Most of these arguments disintegrate upon closer scrutiny. The nation's interest is becoming clearer each day as the stream of refugees continues to flow across Syria's borders. U.S. allies such as Turkey and Iraq are being impacted by this stream, which the United Nations now says equals 2 million Syrians living outside their own country. Israel, the main U.S. ally in the region, is perhaps the most vulnerable to any nearby conflict in a neighborhood where political balances are fragile and tied to much larger global powers.

The United States and its allies are unable to respond to every atrocity committed by a dictator, but it makes sense to draw a firm line on the widespread use of chemical weapons, whose effects are especially cruel and indiscriminate. While there is evidence Assad has used such weapons in a limited capacity before, the latest attack defies earlier warnings from the United States and resulted in the deaths of what the administration said was more than 1,400 people, including at least 400 children.

While it is true the United States did not respond when Hussein used chemical weapons on his own people, it is significant to note Hussein's actions eventually forced the United States into two wars. Dictators willing to gas their own citizens tend to be capable of other atrocities, as well.

The Syrian situation is complicated by a rebel force that includes people with troubling ties to al Qaida, and whose intentions for governing Syria are unclear. That makes a limited military action necessary, even though it might change the course of the conflict.

Sen. John McCain, R-Ariz., was right when he said a congressional rebuff to a presidential request for military action would be "catastrophic" for the nation's credibility. The United States is not just any other nation. It is the world's best hope for freedom and liberty.

This is the case the administration must make to reluctant lawmakers in both parties.