President Obama said Tuesday he would seize one last diplomatic opening to avoid military strikes on Syria but made a forceful case for why the United States must retaliate for its alleged use of chemical weapons if the effort fails.

In a nationally-televised address from the White House, Obama cautiously welcomed a Russian proposal that the government of Syrian President Bashar al-Assad give up its entire stockpile of chemical weapons, signaling that he would drop his call for a military assault on the regime if Assad complies.

But with little guarantee that diplomacy would prevail, Obama argued that the nation must be prepared to strike Assad. Facing a skeptical public and Congress, the war-weary president said the United States still carries the burden of using its military power to punish regimes that would flout long-held conventions banning the use of biological, chemical and nuclear weapons.