WASHINGTON – Syrian activists are accusing President Bashar Assad’s forces of a nerve gas attack that killed hundreds, possibly more than 1,000 people Wednesday — many of them children — in the Damascus suburbs. Horrific photos and video have emerged, showing the purported victims. If the reports are verified, this would be by far the worst use of chemical weapons in the current conflict.

The timing of the attack also seems deliberately provocative. A U.N. team is currently in Syria investigating allegations of chemical attacks by both sides in the war. (Syrian state television has been quick to blame the reports on rebels trying to distract the inspectors.) It is also almost exactly a year since U.S. President Barack Obama announced at the White House 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 government 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 from U.S. diplomats in the region were sketchy and allowed room for plausible deniability in Washington. Over time, reports of the use of sarin and other nerve agents by government forces became more frequent and the casualty figures grew.

Gradually the use of chemical weapons in the war appears to have become regularized, and the window of what the international community was willing to tolerate was pushed open a little bit at a time. (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.

This aid has been slow to arrive, and in any case is unlikely to be enough to tip the military balance in the conflict. There are reports that the administration is now increasingly skeptical of providing even limited military aid to the rebels. U.S. public opinion has been consistently opposed to U.S. involvement in the Syrian conflict.

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 this 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 E. Keating is a staff writer for Slate focusing on international news.