(CNN) -- What's happening now in Iraq is dramatic, significant, quite possibly historic. But, to some, it is not surprising.
Militants believed to be from the Islamic State in Iraq and Syria, also known as ISIS, overmatched government forces and now control a vast swath of its territory. Hundreds of thousands have fled, becoming refugees overnight. Sectarian violence plagues some areas not under ISIS control.
And amid all this, some believe the Baghdad-based central government won't be able to do much about it.
Some of these developments, like the fall of Mosul, have been swift and sudden. Others, such as the tensions between Shiite and Sunni Muslims, date back for decades, if not centuries.
But all this trouble didn't come out of nowhere. For years, experts have predicted that various factors -- some rooted in history, some of them related to recent big decisions, some functions of what's happening in the region -- could foster instability and violence in Iraq.
PREDICTION: Breaking up the army after Saddam Hussein's fall could haunt Iraq
While not as big as what it had been prior to the 1991 Gulf War, Iraq's military under Saddam Hussein boasted an estimated 430,000 soldiers and another 400,000 personnel in paramilitary units and security services when U.S.-led troops invaded in spring 2003.
Still, the Iraqis proved no match for coalition forces.
After the military was overrun, it was dissolved -- along with Iraq's defense and information ministries -- by Paul Bremer, the top U.S. civilian administrator in Iraq.
That left hundreds of thousands of troops suddenly out of work. Those with ranks of colonel and above -- who knew the most about strategy, tactics and more -- were hit even harder, as they weren't entitled to severance packages and couldn't work for the new Iraqi government.
Then they had to go somewhere.
According to Fawaz Gerges, a professor at the London School of Economics and Political Science, "hundreds, if not thousands, of skilled officers of Saddam Hussein's ... joined ISIS."
That means this militant force -- even as it is supplemented by foreign fighters -- is trained and knows Iraq well. And its leaders may be more organized, strategically savvy and adept at fighting than some in Iraq's current military.
"(This) has allowed ISIS to basically have skills, to have motivation, to have command and control," Gerges told CNN. "It's a mini-army fighting both in Iraq and Syria."
PREDICTION: Syria's civil war will destabilize the region
ISIS is not just fighting, it's winning.
As of Wednesday, the militant group had taken over not just Iraq's second-largest city in Mosul but also Tikrit (which is Saddam's hometown). It has a major presence in northeastern Syria.
That latter fact speaks to how ISIS -- after the group it emerged from, al Qaeda in Iraq, suffered heavy losses in the 2000s -- was able to emerge as a significant, and stronger, fighting force.
This growth is thanks, in large part, to the success that ISIS has had in Syria since 2011, when that country's civil war began.
In that time, the militant group gained experience, recruits and resources as it gobbled up territory -- and, with it, millions of dollars and military firepower that has helped it to flourish.
And Ramzy Mardini, a fellow at the Atlantic Council think tank now in Jordan, says there's been a "concerted effort to merge Iraq and Syria into one sectarian theater," crediting what's happened in Syria with breathing "new life into militancy in Iraq, rejuvenating their confidence, resources and cause."
"And they have an agenda," adds CNN's Nic Robertson, referring to ISIS's goal to create a caliphate -- or an Islamic state -- across a vast area that includes Syria and Iraq. "We're now seeing ... that agenda (in action)."
British Foreign Secretary William Hague agrees that the Syrian conflict has negatively affected other countries, such as Iraq. That is one major reason why, he says, some accord needs to be reached in Syria -- though that won't solve everything.
"(The regional unrest) underlines the importance of renewed efforts over the coming months to bring about a political solution in Syria," Hague told CNN's Christiane Amanpour. "But in Iraq ... it is also vital that there is some political progress that helps the Iraqi government respond effectively to this."
PREDICTION: Divisions between Shiite, Sunni Muslims cannot be overcome
By "political progress," Hague was likely referring to the sectarian divide that defines Iraqi politics and government. The latter is led by Prime Minister Nuri al-Maliki, a Shiite Muslim. Sunni Muslims -- the minority in Iraq -- often find themselves left out.
"The single biggest error" after the 2003 tippling of Saddam Hussein was not making reconciliation a higher priority, according to Mardini.
Even then, James Jeffrey, U.S. ambassador to Iraq from 2010 to 2012, doesn't think that it was inevitable Shiite and Sunnis in Iraq would be at loggerheads, as they are now.
The veteran diplomat, now a fellow at The Washington Institute for Near East Policy think tank, says that "Sunnis by and large had bought into the new Iraq." They served in the Iraqi military, some of them as generals.
But in the last few years, relations have gone downhill. Jeffrey points to what's happened in Syria, issues with Iraq's military and the fact that both the military and the government are increasingly divided along sectarian lines.
"There was the atrocious political campaign against Sunnis ... and simply a lack of effort to bring Sunnis into the government," said the former ambassador.
Well before the recent events in Mosul, this sectarian strife has manifest itself in violence. About 8,000 civilians were killed in 2013, in what the United Nations says was the deadliest year for civilians since 2008.
Outright violence isn't the only problem. It's also a matter of trust. To that end, it's hard to state how much animosity some Sunnis have for al-Maliki and his government. And ISIS has tried to take advantage, Gerges explained.
"The rift has allowed ISIS to basically position itself as the spearhead of the Sunni community," Gerges said. "It has found a ... refuge and a social base. That particular social base among the Sunni community has allowed ISIS to gain momentum and recruits."
Granted, this momentum could be stopped.
Gerges said doing so will require convincing the vast majority of Sunnis that they are better off with a Baghdad-based government -- maybe one that's more of a unity government, with more Sunni inclusion -- than living under radical Islamist militants.
But picking which is the lesser of two evils for Sunnis may not be simple.
"The Sunni population does not want to be dominated by (ISIS); they went through that in 2004," said Jeffrey, the former American ambassador. "But by the same token, they are disillusioned with the Iraqi government in Baghdad."
PREDICTION: The United States will regret pulling out of Iraq
So what can be done to stop militants from a group that even al Qaeda has disowned, for its excessive and abusive actions in Syria, from taking over Iraq?
Jeffrey doesn't think offering amnesty to some Sunni fighters or forming a unity government will do much to seriously "change the dynamic" overall.
The landscape could be altered, he says, by military intervention by Turkey (which might have special motivation given the conflict's tight proximity to its border and the fact militants seized dozens at its consulate in Mosul) or by Iran (which is a Shiite state, and thus has a natural alliance with al-Maliki).
Then, of course, the U.S. military could act.
It's done so before, sending tens of thousands of troops into the Middle Eastern nation during the 2003 invasion. While units rolled in and out, the military stayed there for years -- peaking at 166,300 troops in October 2007, according to the Defense Department.
But they were all gone by the start of 2012, fulfilling a vow made by President Barack Obama.
That move drew sharp criticism in some circles. Michael Hayden, the National Security Agency director from 1999 to 2005 who led the CIA in President George W. Bush's last 2½ years in office, characterized the withdrawal as a mistake that may undermine the country and the region.
"Whatever the withdrawal means in purely physical terms in Iraq," Hayden wrote in a CNN.com op-ed, "the psychic impact there and in the region is that America is less interested. In Iraq that means that each of the factions are going to their sectarian corners and are preparing to come out fighting."
It's not like the United States cut ties with Iraq entirely or that it's backing away more now.
A Defense Department official says about $15 billion in equipment, training and other services, and firepower including helicopter-fired rockets, machine guns, grenades and rifles have gone to Iraq. Another $1 billion in arms and goods, including up to 200 Humvees, are in the works.
"We are committed to ensuring that ammunition and equipment Iraq needs in its current fight are delivered as quickly as possible," the official said.
A U.S. official said the Iraqi government has indicated a willingness for the United States to conduct airstrikes to help repel insurgents.
But there has been no indication that American troops will be back to fighting on the ground in Iraq anytime soon.
PREDICTION: Iraq won't stay together, over the long term
Without combat intervention from another country, saving Iraq -- as it's currently constituted -- is up to its military and government.
Will they be up to the task?
Recent developments aren't particularly encouraging. Witnesses reported Iraqi security forces abandoned their posts in droves in Mosul, some of them dropping their weapon, taking off their uniforms and running.
And, of course, militants have had success in many other parts of the country, with government fighters sometimes offering sparse resistance.
Jeffrey said better training of Iraq's military, more effective air power and better counterterrorism capabilities may have stopped, or at least slowed, the ISIS onslaught. But now, the longtime diplomat said, Iraq's military is "ill-trained, badly led and not particularly competent."
"They clearly cannot fire and maneuver," said Jeffrey, a U.S. Army veteran.
The Baghdad-based government to which these troops report doesn't inspire confidence in some circles, either.
As CNN's Robertson notes, it's challenged enough "putting out ... fires all over the country, never mind rallying troops to Mosul."
Mardini, from the Atlantic Council, says intelligence, logistics and communication are lacking, though not all these deficiencies can explain the Iraqi security forces' performance recently.
"Iraq is a million-man police state," he said. "It has manpower and capabilities, but it lacks in discipline and professionalism."
Plus, the government's frayed relations with anyone who is not Shiite -- be they Sunnis or Kurds, in the semiautonomous region in Iraq's northeast -- make creating a united front difficult.
Part of the challenge is that many Iraqi feel more loyalty to their groups -- be they Shiite, Sunni or Kurd -- than to Iraq as a whole. No other than U.S. Vice President Joe Biden proposed, in a 2006 joint New York Times editorial, that one way to keep Iraq united is by "giving each ethno-religious group ... room to run its own affairs."
For all the doubts, Iraq's government believes that the country can remain as one. And the military, it says, will be a big reason why.
"This is not the end," the Defense Ministry vowed in a statement. "We are very confident that we will be able to correct the path and to overcome mistakes."