Syrian activists are accusing President Bashar Assad's forces of a nerve-gas attack that killed hundreds, possibly more than 1,000 people, last Wednesday - many of them children - in the Damascus suburbs. Horrific photos and video have emerged, showing the purported victims. This would be by far the worst use of chemical weapons in the conflict.

It is also almost exactly a year since U.S. President Barack Obama announced that he would seek to avoid military engagement in Syria but that "a red line for us is (if) we start seeing a whole bunch of chemical weapons moving around or being utilized."

Picking this in particular as the line that shall not be crossed was always a bit odd and was seen by some at the time as a carte blanche for Assad to continue massacring citizens as long as he stuck to conventional weapons. But the degree to which Assad consistently has been able to get away with the one thing the international community specified he must not do is still striking.

He's done this - as he's conducted the rest of the war - by escalating the level of carnage bit by bit. Remember that Obama's justification for the international operation that led to the downfall of Libyan leader Moammar Gadhafi was that the United States and its allies "intervened to stop a massacre." At the time, the rebel stronghold at Benghazi - a city of more than 600,000 people - was surrounded by government forces, and massive bloodshed appeared imminent if no action were taken. The war in Syria has now killed more than 100,000 people, but the escalation has been slow, with mostly small attacks rather than attention-grabbing massacres.

To be blunt, if the Syrian military had launched a massive chemical attack that killed hundreds of people including dozens of children last September, the U.S. and European governments might have felt compelled to respond with military force or at least a major escalation in military aid. Instead, Assad has slowly pushed the envelope.

The first reports of Syrian chemical-weapons use were sketchy and allowed room for plausible deniability in Washington. (And it should be noted that chemical-weapons allegations have been leveled against the rebels as well.)

It was only last April, when the reports became essentially undeniable, and U.S. allies were discussing them openly, that the Obama administration felt it had to rethink its position. In June, the United States announced it would begin providing military aid to the rebels.

But there are reports that the administration is now increasingly skeptical of providing even limited military aid to the rebels. U.S. public opinion also has been consistently opposed to U.S. involvement.

The skepticism on behalf of both the White House and the public is more than understandable. But it's still tempting to wonder if an attack like last week's might have shifted the balance if it had happened in the early days of the conflict. After two years of gradually escalating bloodshed, Syria is a part of the world Americans and their leaders now feel comfortable ignoring.

Assad likely knows this, which is why he's getting away with it.

Joshua Keating is a staff writer at Slate.