Iraqi forces are now alone in facing militant groups revitalised by widespread discontent among the minority Sunni Arab community, which complains of being marginalised and targeted by Shiite-led authorities, and by the brutal war in neighbouring Syria.
They have shortcomings that experts say range from a decline in training and intelligence capabilities to politicisation.
And they no longer have ready access to US expertise, firepower and support that they could fall back on in the past.
Iraqi forces have also been repeatedly accused of carrying out abuses including torture.
Iraq is now hit by daily attacks -- bombs rip through cafes, mosques, markets, weddings and funerals, people are gunned down, and security forces and officials are frequently targeted.
The violence has killed more than 6,500 people since the beginning of 2013, raising questions about the ability of Iraqi forces to secure the country.
"US forces were overseeing or participating or coordinating with the Iraqi forces in their missions before the withdrawal," a senior Iraqi army officer told AFP.
"Iraq is still at the beginning of the road," said the officer, who spoke on condition of anonymity, adding that the US withdrawal "made us take responsibility before we completed filling the shortfall".
The US officially ended combat operations in Iraq in 2010, shifting its focus to training Iraqi forces.
The following year, negotiations on a post-2011 US training mission stalled when Iraq refused to grant US forces legal immunity and Washington declined to keep troops in the country without it.
The last American military personnel, except for a small number under US embassy authority, left Iraq on December 18, 2011.
"One result was that we departed without finishing many basic training goals," said Frank Helmick, a retired US army lieutenant general who served multiple tours in Iraq, including in 2011.
"Additionally, the Iraqi air force was not yet ready to defend its sovereign airspace and still does not possess that capability," Helmick told AFP.
"Finally, the (Iraqi security forces) relied on the American military -- in conjunction with US and Iraqi special operations forces -- for the intelligence support that allowed them to sustain pressure on insurgent networks," he said.
"That capability has suffered in the absence of direct American support."
And Iraqi forces face various other shortcomings, including in maintenance and sustainment, integration of army and police forces capabilities, tactical communications and external defence, Helmick said.
He said he does believe they have the required basic skills.
James Jeffrey, the US ambassador to Baghdad from 2010 to 2012, said training seems to have declined since US forces left.
"We had a very sophisticated programme that we were carrying out while our troops were there... training their companies, battalions and brigades, and they're not doing (that), as far as I can see, or they're not doing it to the same degree," he told AFP.
A post-2011 US training mission in Iraq would have helped the country's forces hone key skills they still need to develop, he said.
"Deploying a military force is extremely complicated ... when people are shooting at you, and it requires constant training, it requires a lot of expertise -- they don't have that yet, and we could've given it to 'em," said Jeffrey.
'Repeating Saddam's efforts'
A Centre for Strategic and International Studies (CSIS) report on Iraq's security forces said the country "has not been able to find effective ways to replace its past dependence" on US assistance.
It also identified Prime Minister Nuri al-Maliki's consolidation of control over the armed forces as an issue, saying his office "controls key military and paramilitary forces directly, Iraqi intelligence, and national elements of the police and some elements of Iraq's judiciary".
And it noted that "Iraq's political leadership (has) insisted on repeating Saddam Hussein's efforts to micromanage every aspect of security operations, enforce political control, bypass the formal chain of command, and limit initiative".
Ultimately, shortcomings in the Iraqi security forces make it more difficult to curb the rampant violence, but there are deep-seated political issues, especially Sunni discontent, fuelling the unrest.
Iraq needs to "forge a better level of national understanding" and "move toward a more national government", Anthony Cordesman, the primary author of the CSIS report, told AFP.
Otherwise, "it is going to find itself, as it already is, returning more and more to the kind of civil war that existed back in the mid-2000s".