Updated: Nov 11, 2018
The toppling of a brutal dictatorship? The enabling of Daesh? Bush's Folly?
The Bush administration based its rationale for the war principally on the assertion that Iraq, which had been viewed by the U.S. as a rogue state since the 1990-1991 Gulf War, possessed weapons of mass destruction(WMDs) and that the Iraqi government posed an immediate threat to the United States and its coalition allies. Select U.S. officials accused Saddam of harboring and supporting al-Qaeda, while others cited the desire to end a repressive dictatorship and bring democracy to the people of Iraq. After the invasion, no substantial evidence was found to verify the initial claims about WMDs, while claims of Iraqi officials collaborating with al-Qaeda were proven false.
The rationale and misrepresentation of U.S. prewar intelligence faced heavy criticism both domestically and internationally, with President Bush declining from his record-high approval ratings following 9/11 to become one of the most unpopular presidents in U.S. history. From 2009-2011, the UK conducted a broad inquiry into its decision to go to war chaired by Sir John Chilcot. The Chilcot Report, published in 2016, concluded military action may have been necessary but was not the last resort at the time and that the consequences of invasion were underestimated. 
Finishing Where the Coalition Left Off?
A mere 14 months prior to the start of the Iraq War--Operation Iraqi Freedom--I had returned home aboard USS CARL VINSON (CVN 70) following the initial strikes and three months of combat operations at the start of Operation Infinite Justice/Enduring Freedom to deliver America's response to the 9/11 attacks against al-Qaeda positions and free Afghanistan from the brutal Taliban regime. Operation Iraqi Freedom started on March 20, 2003...
According to General Tommy Franks, there were eight objectives of the invasion:
"First, ending the regime of Saddam Hussein.
Second, to identify, isolate, and eliminate Iraq's weapons of mass destruction.
Third, to search for, to capture, and to drive out terrorists from that country.
Fourth, to collect such intelligence as we can relate to terrorist networks.
Fifth, to collect such intelligence as we can relate to the global network of illicit weapons of mass destruction.
Sixth, to end sanctions and to immediately deliver humanitarian support to the displaced and to many needy Iraqi citizens.
Seventh, to secure Iraq's oil fields and resources, which belong to the Iraqi people.
And last, to help the Iraqi people create conditions for a transition to a representative self-government." 
Legality of the invasion
Human rights violations such as the Iraq prison abuse scandals
Insufficient post-invasion plans, in particular inadequate troop levels (a RAND Corporation study stated that 500,000 troops would be required for success)
Financial costs with approximately $612 billion spent as of 4/09 the CBO has estimated the total cost of the war in Iraq to the United States will be around $1.9 trillion.
Adverse effect on U.S.-led global "war on terror"
Damage to U.S.' traditional alliances and influence in the region, especially Israel and Saudi Arabia.
Endangerment and ethnic cleansing of religious and ethnic minorities by insurgents
Disruption of Iraqi oil production and related energy security concerns (the price of oil has quadrupled since 2002)
Fallujah and the Marines
Two battles for Fallujah taxed the Marines Corps greatly and resulted in some of the most ferocious and deadly fighting of the Iraq War.
The First Battle of Fallujah
In response to the killing of the four US citizens, and intense political pressure, the US Marines commenced Operation Vigilant Resolve. They surrounded the city and attempted to capture the individuals responsible as well as others in the region who might have been involved in insurgencies. One out of every two mosques in Fallujah were used to hide fighters or weapons. The Iraqi National Guard was supposed to work alongside the US Marines in the operation, but on the dawn of the invasion they discarded their uniforms and deserted. Under pressure from the Iraqi Governing Council, the US aborted its attempt to regain control of Fallujah. The US Marines suffered 40 deaths in the siege. Estimates of the number of Iraqi deaths (both fighters and civilians) in the attack range from 271 (according to Iraqi Ministry of Health officials) to 731 (according to Rafie al-Issawi, the head of the local hospital).
The Second Battle of Fallujah
On November 8, 2004, a force of around 2,000 U.S. and 600 Iraqi troops began a concentrated assault on Fallujah with air strikes, artillery, armor, and infantry. The New York Times reported that within an hour of the start of the ground attack, troops seized the Fallujah General Hospital. "Patients and hospital employees were rushed out of rooms by armed soldiers and ordered to sit or lie on the floor while troops tied their hands behind their backs". Noam Chomsky in his book Failed States commented that according to the Geneva Conventions, medical establishments "may in no circumstance be attacked, but shall at all times be respected and protected by the Parties to the conflict." Troops seized the rail yards North of the city, and pushed into the city simultaneously from the North and West taking control of the volatile Jolan and Askari districts. By nightfall on November 9, 2004, the U.S. troops had almost reached the heart of the city. U.S. military officials stated that 1,000 to 6,000 insurgents were believed to be in the city, they appear to be organized, and fought in small groups, of three to 25. Many insurgents were believed to have slipped away amid widespread reports that the U.S. offensive was coming. During the assault, Marines and Iraqi soldiers endured sniper fire and destroyed booby traps, much more than anticipated. Ten U.S. troops were killed in the fighting and 22 wounded in the first two days of fighting. Insurgent casualty numbers were estimated at 85 to 90 killed or wounded. Several more days of fighting were anticipated as U.S. and Iraqi troops conducted house-to-house searches for weapons, booby traps, and insurgents.
On November 16, 2004, a Red Cross official told Inter Press Service that "at least 800 civilians" had been killed in Fallujah and indicated that "they had received several reports from refugees that the military had dropped cluster bombs in Fallujah, and used a phosphorus weapon that caused severe burns."
As of November 18, 2004, the U.S. military reported 1,200 insurgents killed and 1,000 captured. U.S. casualties were 51 killed and 425 wounded, and the Iraqi forces were 8 killed and 43 wounded.
On December 2, 2004, the U.S. death toll in Fallujah operation reached 71 killed.
Some of the tactics said to be used by the insurgents included playing dead and attacking, surrendering and attacking, and rigging dead or wounded with bombs. In the November 13th incident mentioned above, the U.S. Marine alleged the insurgent was playing dead.
Of the 100 mosques in the city, about 60 were used as fighting positions by the insurgents. The U.S. and Iraqi military swept through all mosques used as fighting positions, destroying them, leading to great resentment from local residents.
In 2005, the U.S. military admitted that it used white phosphorus as an anti-personnel weapon in Fallujah.
Residents were allowed to return to the city in mid-December after undergoing biometric identification, provided they carry their ID cards all the time. US officials report that "more than half of Fallujah's 39,000 homes were damaged, and about 10,000 of those were destroyed." Compensation amounts to 20 percent of the value of damaged houses, with an estimated 32,000 homeowners eligible, according to Marine Lt. Col. William Brown. According to the NBC, 9,000 homes were destroyed, thousands more were damaged and of the 32,000 compensation claims only 2,500 had been paid as of April 14, 2005. According to Mike Marqusee of Iraq Occupation Focus writing in The Guardian,"Falluja's compensation commissioner has reported that 36,000 of the city's 50,000 homes were destroyed, along with 60 schools and 65 mosques and shrines". Reconstruction is only progressing slowly and mainly consists of clearing rubble from heavily damaged areas and reestablishing basic utility services. This is also due to the fact that only 10% of the pre-offensive inhabitants had returned as of mid-January, and only 30% as of the end of March 2005.
America's Black Eye: Abu Ghraib
 Personnel of the United States Army and the Central Intelligence Agency committed a series of human rights violations against detainees in the Abu Ghraib prison in Iraq. These violations included physical and sexual abuse, torture, rape, sodomy, and murder. The abuses came to widespread public attention with the publication of photographs of the abuse by CBS News in April 2004. The incidents received widespread condemnation both within the United States and abroad, although the soldiers received support from some conservative media within the United States.
The administration of George W. Bush asserted that these were isolated incidents, not indicative of general U.S. policy. This was disputed by humanitarian organizations such as the Red Cross, Amnesty International, and Human Rights Watch. These organizations stated that the abuses at Abu Ghraib were not isolated incidents, but were part of a wider pattern of torture and brutal treatment at American overseas detention centers, including those in Iraq, Afghanistan, and Guantanamo Bay. Several scholars stated that the abuses constituted state-sanctioned crimes.
The United States Department of Defense removed seventeen soldiers and officers from duty, and eleven soldiers were charged with dereliction of duty, maltreatment, aggravated assault and battery. Between May 2004, and March 2006, these soldiers were convicted in courts-martial, sentenced to military prison, and dishonorably discharged from service. Two soldiers, Specialist Charles Graner and PFC Lynndie England, were sentenced to ten and three years in prison, respectively. Brigadier General Janis Karpinski, the commanding officer of all detention facilities in Iraq, was reprimanded and demoted to the rank of colonel. Several more military personnel who were accused of perpetrating or authorizing the measures, including many of higher rank, were not prosecuted. It was reported that most inmates were innocent of the crimes they were accused of and were simply detained due to their being in the wrong place at the wrong time.
Documents popularly known as the Torture Memos came to light a few years later. These documents, prepared shortly before the 2003 invasion of Iraq by the United States Department of Justice, authorized certain enhanced interrogation techniques, generally held to involve torture of foreign detainees. The memoranda also argued that international humanitarian laws, such as the Geneva Conventions, did not apply to American interrogators overseas. Several subsequent U.S. Supreme Court decisions, including Hamdan v. Rumsfeld (2006), have overturned Bush administration policy, and ruled that Geneva Conventions apply.
Many of the torture techniques used were developed at Guantánamo detention center, including prolonged isolation; the frequent flier program, a sleep deprivation program whereby people were moved from cell to cell every few hours so they couldn't sleep for days, weeks, even months, short-shackling in painful positions; nudity; extreme use of heat and cold; the use of loud music and noise and preying on phobias.
Synoptic lineup of results of the Iraq War :
Invasion and occupation of Iraq
Overthrow of Ba'ath Party government and execution of Saddam Hussein
Emergence of significant insurgency, rise of al-Qaeda in Iraq, severe sectarian violence
Subsequent reduction in violence and depletion of al-Qaeda in Iraq
Establishment of democratic election sand formation of new Shia led government
Withdrawal of U.S. forces from Iraq in 2011
Stronger Iranian influence in Iraq
Escalation of sectarian insurgency after U.S. withdrawal leading to the rise of the Islamic State of Iraq and the Levant (ISIL), the successor of al-Qaeda in Iraq
Iraqi Civil War (2014–2017)
Return of U.S. forces to Iraq in 2014
Subsequent end of the Islamic State of Iraq and the Levant in Iraq in 2017
Editor's Note: During the time when Operation Iraqi Freedom--i.e. The Iraq War--started, I was assigned as Senior Naval Officer at NORAD's Western Air Defense Sector Headquarters at what is now Joint Base Lewis-McChord. In 2006, I joined the faculty of Air War College outside Montgomery, Alabama--during the next five years, I met and became friends with some of the Iraqi Air Force's senior officers and their families both of which went on to become General Officers in their country's Air Force. My last year before retirement, I met the first Afghanistan Air Force officer to attend our senior military education college--LtCol Mohammed Sapy. These exceptional men and their famiolies helped manage their countries through the transition periods of both Iraqi Freedom (in Iraq) and Enduring Freedom (in Afghanistan). They have my enduring respect and friendship.
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Also: Fallujah during the Iraq War. Wikipedia. https://en.wikipedia.org/wiki/Fallujah_during_the_Iraq_War