Iraq at risk again: How did we get here so fast?

It's crumbling so quickly: Just 2 1/2 years after American troops came home, Iraq is back in crisis. And chaos in Iraq, a diverse nation that stands as a buffer zone between the mostly Sunni Mideast and mostly Shiite Iran, is troubling around the world.

There were plenty of warnings, of course. Here's a look at how we got here:

When did the trouble start?

The answer depends: 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 Muhammad and a dispute over who should succeed him as leader of the Muslims. 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. Through wars and upheaval, the national borders they drew have pretty much held, largely by the force of autocratic rulers.

2003: A U.S.-led coalition invaded Iraq and deposed Saddam Hussein, and mayhem broke out. Saddam had ruthlessly held the nation together for more than two decades, favoring his fellow Sunnis while wiping out multitudes of Shiites and Kurds. Americans, flush with the fervor that followed the Sept. 11 terrorist attacks, hoped to replace him with a friendly democracy. They met waves of bombings, massacres and kidnappings in sectarian fighting that peaked in 2006 and 2007, when additional U.S. troops began arriving and helped to temporarily tamp down the violence.

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 between Shiites, Sunnis and Kurds and heal the national wounds. It didn't work out. 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 northern Iraq.

2013: The situation in Iraq began deteriorating rapidly. Sunni protesters took to the streets, al-Qaida-inspired militants stepped up their attacks, and fighting from Syria's civil war spilled over the border into Iraq.

ISIS, ISIL: Who they are, what they want

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 U.S. State Department classified al-Qaida in Iraq as a terrorist organization in 2004.

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.

Leaders of the core of al-Qaida objected to the group's attacks on fellow Muslims in Iraq, worrying that would hurt the larger cause of jihad against the West.

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

Their name is sometimes translated from Arabic as the Islamic State of Iraq and Syria. That understates the relatively small group's outsized ambition.

It wants to create an Islamic state ruled by Shariah 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 Shariah orders women to stay inside their homes, bans music and punishes thieves by cutting off their hands.

What's the Syrian connection?

As the U.S. was winding down operations in Iraq in 2011, the Arab Spring protests were underway.

Uprisings forced out the rulers of Tunisia, Egypt, Libya and Yemen. In Syria, President Bashar Assad's deadly crackdown on demonstrators sparked a civil war, with no end in sight.

It's a rebellion that reverberates strongly among Iraq's Sunnis: The Syrian rebels are mostly Sunnis, fighting a repressive government dominated by Alawites, an offshoot of Shiite Islam.

The bloodshed in Syria revitalized the flagging Islamic State.

The extremist group joined the fight and began gathering new recruits from among the foreigners pouring in to battle Assad. It set up operations in Syria that serve as a base for the Iraq campaign. It took over a swath of Iraqi and Syrian borderland and turned it into a seedbed for the Islamic State's vision of a caliphate under strict Islamic law.

American intelligence officials worry that this fiefdom could be used to train jihadis with Western passports to attack the United States.

The United States also wants Assad out of Syria. But it is limiting its assistance to Syrian rebels to avoid helping extremists such as the Islamic State.

The group's land grabs, brutality and extreme religious rules alienated even some of its would-be allies on the ground. Heavy fighting for the past six months between the Islamic State and other insurgents has weakened the Syrian opposition.

Is Baghdad on the brink?

The Islamic State's bold and bloody sweep through northern and western Iraq this year 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?

For one thing, it's the home team. 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.

The Iraqi army is awash in corruption, desertion and battered morale. Many Sunni and Kurd soldiers and police feel little loyalty to Baghdad; they balk at fighting and killing their own people. When Islamic State fighters moved into Mosul, a security force of some 75,000 troops and police collapsed and scattered.

The Islamic State also appears to be getting help from Sunni tribes and elements of Saddam's old Baath Party. The insurgents rolled into Saddam's hometown of Tikrit without opposition.

Despite its threats, the Islamic State 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.

Where does Iran come in?

Maybe on the American side, for a change.

The list of complaints between the United States and Iran is long and grievous: The U.S. accused Iran of aiding Shiite militias that killed American troops during the Iraq War. It says Iran sponsors global terrorists and poses a potential nuclear threat to Israel.

The two nations stand opposed on Syria, where Iran is propping up Assad.

Yet when it comes to Iraq, their interests align, at least in the short term. Both Iran and the U.S. want stability in Iraq; they share a common foe in the Islamic State.

There's also grim history behind Iran's alliance with a Shiite-controlled Iraq.

When Saddam and Sunnis ran Iraq, they invaded Iran and started an eight-year war that cost a million lives. Back then, the United States supported Saddam over Iran's Ayatollah Ruhollah Khomeini, who held Americans hostage and called the U.S. the "Great Satan."

Today, mostly Shiite Iran badly wants an ally instead of an enemy across the border in Iraq.

President Barack Obama's administration has made overtures to Iran about possibly working together on the problem. Obama says that means teaming up to persuade Iraq's government to make amends with the Sunnis and Kurds -- not military cooperation.

But Iran already is helping Iraq strengthen its military and Shiite militias to battle the militant Sunni onslaught.

Across the Middle East, that could look like another omen of a spreading religious war.