Barack Obama has committed the US to a protracted air war to stop the Islamic State from overrunning Iraq, as America launched a third round of air and drone strikes.
He is the fourth straight US president to bomb Iraq - the last president to not commit to such an action was Ronald Reagan.
Obama has said the US military campaign is likely to be "long term", with aid drops insufficient to remedy an ever-worsening humanitarian situation, especially with Islamic State fighters feared to be on the brink of taking Erbil, the capital of Iraqi Kurdistan. Kurdistan's peshmerga fighters have so far presented the only defence against jihadist fighters.
I don't think we're going to solve this problem in weeks. All Iraqi communities are ultimately threatened by these barbaric terrorists.
- Barack Obama
Britain is involved in the humanitarian side of the operation, with £8million worth of UK emergency aid leaving RAF Brize Norton in Oxfordshire yesterday. France is also delivering aid supplies.
Downing Street admits the aid drops are not a "long-term solution", with supplies not enough to prevent deaths from starvation among thousands from the Yazidi minority cut off in the mountains around Sinjar.
The air and drone strikes have been limited so far, hitting an IS convoy and artillery stations, but they have boosted morale among Kurdish troops and residents in Erbil.
The west still sees the beleaguered government in Baghdad as the only authority able to ultimately defeat IS, but the plight of the Yazidis, Christians and other minorities led to military invention on a humanitarian basis.
The Kurdistan Regional Government's peshmerga fighters outnumber IS by about three to one, but while they are thought to be well-disciplined they are comparatively poorly-armed and have had little combat experience in recent years.
The Islamists, already battle-hardened in Syria, fight with a fanaticism that has given them stunning territorial gains they themselves attribute to divine intervention. More to the point, they are armed with state-of-the-art US-made weaponry captured from fleeing Iraqi troops. In short, the US is bombing its own weapons.
The desperation of the situation was underlined on Sunday when Iraq's human rights minister told the Reuters news agency 500 Yazidis has been killed by IS. He claimed that women and children had been buried alive, and 300 women had been taken as slaves.
More than half a million people are thought to have been displaced since IS, then known as the Islamic State in Iraq and the Levant (Isis), captured Mosul when the Iraqi army simply melted away.
Mount Sinjar, a rocky plateau of baked earth and jagged rocks, has been home to tens of thousands of Yazidi civilians who fled IS when presented with the ultimatum of convert to Sunni Islam or die. Thousands have now been safely evacuated, but not before many starved to death.
Obama presaged the announcement of his military intervention by accusing IS of threatening a genocide against Iraqi minorities.