The limited use of military force announced by President Obama was likely prompted by concern at the success of ISIS's latest offensives across Syria and Iraq. The jihadist group has recently redoubled its efforts in Raqqa, Syria, in an effort to take the remaining pockets of Syrian Government-held territory in the province. At the same time, ISIS's performance against Kurdish forces would have raised concern in Washington and Baghdad, and led to a re-assessment of some overly optimistic judgments about Kurdish capabilities.
The situation facing the Yazidis and the Christians is of grave humanitarian concern. The fact that the refugees are geographically concentrated would have made air support an attractive option for the president. Add to this the fact that the sovereign Iraqi Government invited the intervention, and the stage was set for a dual humanitarian/limited direct military intervention operation to which Obama could agree.
There will of course be accusations that Obama is a hypocrite for intervening in Iraq but not Syria. That argument is simplistic and wrong. If the US is obliged to intervene militarily everywhere there is a humanitarian need, it would never stop intervening. Obama said as much in his speech. He is one of the few US leaders to understand the limits of American power.
Moreover, the situation in Syria is far more complex. To have assisted one side would have meant breaching a nation's sovereignty (no big deal) and potentially assisting the very Islamist forces that pose a security threat to the region and the West (a very big deal). The intervention in Iraq requires Obama to do neither of those things, so the calculus is completely different.
In his speech, Obama was careful to emphasise the need for an Iraqi political solution to the crisis engulfing the country. As long as Maliki remains prime minister, there will be little appetite for substantive US air support. The intriguing question is whether a more politically inclusive Iraqi prime minister might prompt a more robust US response in terms of air support and stand-off weapons.