How did we get here so fast?

Just 2½ years after U.S. troops came home and billions of U.S. dollars spent, Iraq is in chaos. There were plenty of warnings. A look at how we got here:

When did the trouble start?

How far back do you want to go?

A.D. 632: The centuries-old split between the Shia and Sunni denominations dates to the death of the prophet Mohammed and a dispute over who should succeed him. Sunnis are the largest branch of Islam. But Shiites outnumber them in Iraq and make up the overwhelming majority of neighboring Iran.

1916: The uneasy borders dividing the Middle East were set during World War I, when the French and English divvied up the lands of the defeated Ottoman Empire with little regard for religious or ethnic differences.

2003: A U.S.-led coalition invaded Iraq and deposed Saddam Hussein. Mayhem broke out. Saddam had ruthlessly held the nation together, favoring his fellow Sunnis while wiping out multitudes of Shiites and Kurds.

2011: A return to factional warfare has been feared ever since U.S. troops pulled out after nearly nine years in Iraq. Americans urged Prime Minister Nouri al-Maliki to craft a government that would share power among Shiites, Sunnis and Kurds and heal the national wounds. It didn't work. Sunnis complain they are excluded, imprisoned and abused by Al-Maliki's Shiite-dominated government. Kurds have focused on building up their oil-rich autonomous enclave in the north.

Who are those guys?

The alarming dispatches from Iraq often feature a jumble of letters new to many American ears: ISIL, or sometimes ISIS. ISIL stands for the Islamic State of Iraq and the Levant, a Sunni insurgent group. Its previous name is more familiar: Al-Qaida in Iraq. The group emerged during the Iraq war as a major player in the Shiite vs. Sunni violence that threatened to rip Iraq apart along sectarian lines.

The Sunni group famously blew up one of Shiite Islam's holiest shrines, the golden domed Al-Askari mosque in Samarra, in 2006. It uses beheadings and videotaped executions to enhance its reputation for brutality. Al-Qaida leaders objected to the group's attacks on fellow Muslims in Iraq, worrying that would hurt the larger cause of jihad against the West.

ISIL moved into Syria in 2013, two years into that country's uprising. The group changed its name, clashed with other factions and eventually had a falling out with the main Al-Qaida, which formally disavowed it in February.

It wants to create an Islamic state ruled by sharia law in Iraq and in "the Levant," a region stretching from southern Turkey into Egypt, encompassing not only Syria but also Jordan and Israel. The group's extremist brand of sharia orders women to stay inside their homes, bans music and punishes thieves by cutting off their hands.

The Islamic State's bold and bloody sweep through northern and western Iraq belies its relatively small numbers — probably fewer than 10,000 fighters, according to U.S. intelligence estimates.

How could a force that size take Fallujah, site of the biggest battle of the Iraq war, and capture Mosul, the second-largest city in Iraq? Many residents of the Sunni heartland prefer the insurgents to Al-Maliki's government. That might change later if the group begins to enforce its Taliban-style version of Islamic law.

Despite its threats, ISIL probably isn't big enough to overrun Baghdad, the Shiite-heavy capital city of 7 million people, much less conquer all of Iraq.

The real fear is that their campaign will spark a wider Sunni uprising, incite retaliation by Shiite militias and start a full-out religious war that could spread across the Middle East.