Three weeks after Syrian President Bashar al-Assad's forces allegedly launched a chemical weapons attack that killed hundreds of civilians, including children, President Obama on Tuesday evening will address the nation about the U.S.'s plan for a response.

The situation has evolved rapidly over the past two days, leaving it unclear what exactly Obama will say aside from making a general case that there must be some form of American-led response. He is expected to at least endorse the basic idea behind a Russian proposal to have Syria hand over its chemical weapons and sign a chemical weapons ban as an alternative to a military strike, according to NBC.

In a round of TV interviews Monday, Obama said he preferred a diplomatic solution to the issue, but needed to keep a military option on the table to ensure a favorable outcome for the U.S.

In the immediate aftermath of the chemical weapons attack, Obama seemed poised to launch a swift military response against limited targets within Syria. Yet would-be coalition members like Britain soon wound up on the sidelines, leaving Obama with the dangerous option of going it virtually alone. In a surprise move, he then asked Congress to authorize a strike, leading to days of frenzied activity on the Hill.

Then this week, Secretary of State John Kerry tossed out the proposal, quickly picked up by Russia and agreed to by Syria, for Assad to turn over his chemical weapons as a way to avert military retaliation. It remains to be seen whether Russia and Syria's cooperation is serious or merely an attempt to stall an international response.