Syria’s President Bashar Assad needs to be shown that using chemical weapons against his opponents won’t be tolerated. If he has used such weapons — or if he impedes investigation of the scene of the latest atrocity in his country — the United States and its allies should respond with force.

Hundreds died last week in what has been said to be an “undeniable” poison gas attack. President Obama has been cautious about intervening in Syria, and rightly so, but the calculation is changing. Assad is testing the limits. The U.S. and its allies need to shift the conversation in a fundamental way: The purpose of intervention should not be to change the course of the civil war but rather to make clear that the use of chemical weapons will not be tolerated.

France opposed the U.S. invasion of Iraq in 2003 because it didn’t believe there was sufficient proof that Saddam Hussein had weapons of mass destruction. It has now called for force to be used against Syria — with or without U.N. Security Council authorization — if the use of chemical weapons is confirmed. Turkey and other U.S. allies in the region have long demanded firmer action. A coalition is forming and is waiting to be led.

Israel has shown that it’s possible to launch targeted strikes in Syria without destroying air defenses beforehand — a daunting task that Gen. Martin Dempsey, the chairman of the U.S. Joint Chiefs of Staff, has said he’s reluctant to undertake. Planners at the Pentagon are, according to a report in the Wall Street Journal, refining military options that would primarily involve weapons launched from afar such as cruise missiles.

That would be the right level of force. The operation must be designed — and must be seen to be designed — not to topple Assad but to show him and everybody else that use of chemical weapons is a war crime that won’t go unpunished.