Talking about Syria last year, President Barack Obama said "a red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized."
The implication was clear. If Syria uses chemical weapons in the civil war, the United States will have to do something.
Now, the White House says it looks like Syria has used chemical weapons against its own people. So here we are.
What will Obama do in response? Whatever it is, it's time to sit up and take notice, because this news story is moving to another level.
5. What makes chemical weapons a game changer?
Some argue that conventional weapons like guns or bombs also have a massive human toll. They say chemical weapons shouldn't be a turning point for the world to act.
But the White House maintains that they're a game changer.
"The use of chemical weapons is contrary to the standards adopted by the vast majority of nations and international efforts since World War I to eliminate the use of such weapons ... The use of these weapons on a mass scale and a threat of proliferation is a threat to our national interests and a concern to the entire world," White House spokesman Jay Carney said this week.
6. Why didn't the United States just send a bunch of weapons to the opposition when it had the chance?
In June, the United States said it would send the rebels small arms, ammunition and potentially anti-tank weapons. But that was long after the unrest started. Why the delay?
Well, some argue that sending weapons to a region of the world that also contains Islamic extremists is risky business.
Many of the rebel fighters are militants with pro-al-Qaida sympathies, the same stripe of militants America has battled in Iraq and Afghanistan.
The Syria rebels have promised U.S. and European officials that any military weaponry they get won't end up in extremists' hands. But that hasn't quelled criticism from some quarters that arming the rebels is a dangerous risk.
Russian President Vladimir Putin has slammed the decision to arm the opposition. At an economic forum in June, he cautioned -- "Where will those weapons end up?"
7. What's the deal with Russia? Why are they criticizing the U.S.?
Putin has made it clear that Russia and the United States don't see eye to eye when it comes to Syria.
Russia and Syria are longtime allies. For one, just take a look at their weapons deals. Between 2007 and 2010, Russian firms selling weapons to Syria made almost $5 billion.
It would be costly for Russia to end that relationship, analyst Peter Fragiskatos told CNN earlier this year
"Russia's leadership still sees much to lose economically and strategically from cutting Syria loose," Fragiskatos wrote. "Russia sees Syria as another test case for the West's appetite for intervention, and views the danger of U.S. involvement as a direct threat to its own interests."
There are other reasons to suspect Russia will keep supporting Syria. Russia's only naval base in the Mediterranean is on the Syrian coast, and Putin is still upset about NATO's bombing in Libya two years ago that removed Russian ally Moammar Gadhafi from power.
8. What's religion got to do with it?
The al-Assad family is Alawite, a Shiite Muslim offshoot that's one of the minorities in a country that is nearly three-quarters Sunni.
Al-Assad has filled key positions in his government with extended family members, and many of his supporters are Alawites and other minorities who fear what might happen if the Sunnis were to gain power.
Because the Syrian regime is Alawite and the majority of the country is Sunni, there are concerns that Syria could spiral into even more violence.
9. What's the worst that could happen?