Washington failed to prevent the spread of weapons, restore public order in the days after it ousted former Iraqi President Saddam Hussein and to quickly reinstate basic services.
These are the findings in a report drawn up by the influential International Institute of Strategic Studies (IISS).
The London-based IISS released its annual Military Balance report, a survey of military capabilities and defence economics across the world, on Wednesday.
Half a year after US and British tanks rolled into Baghdad, “the situation has hardly improved and small arms and light weapons are readily available to criminals and insurgents alike”, said IISS director John Chipman.
The report noted that the US administration was not prepared for the breakdown in law and order that followed the fall of Hussein.
Washington and its staunch ally London launched war in March against Iraq to overthrow Hussein’s government, which it accused of developing weapons of mass destruction. They have found no such arms yet.
More US soldiers have lost their lives in guerrilla-style attacks than during the war.
Delays in providing food, water, electricity and medical services have quickly exacerbated the situation, "leading to dissatisfaction which started to undermine military success, making the transition to, and consolidation of a state of peace ever more difficult," said the report.
This dissatisfaction has also fed into increased resistance attacks, it noted.
The relationship between Europe and the United States was “severely charred" by European complaints about American unilateralism, said Chipman.
But Europe and Washington have worked together effectively over the alleged threat of Iran’s nuclear enrichment programme, which Chipman claimed could give Tehran a "nuclear weapons breakout option" within a few years.
US forces come under daily attack
Iran insists its nuclear programme is strictly for peaceful means, mainly generating electricity.
The security agenda has also been dominated by the issue of North Korea, from which the United States is demanding a dismantling of its nuclear weapons drive.
There seemed to be "no prospects for dramatic progress in the coming months," despite the parties agreeing in principle to meet, said Chipman.
In its assessment of Europe, the IISS report said defence spending remained flat as a debate over the region's future defence plans continued.
NATO was showing signs of change, taking over command and expanding the role of the International Security Assistance Force (ISAF) in Afghanistan.
The IISS report said that Japan had shown a greater desire to take part in international military operations, while China had reduced the People's Liberation Army (PLA) by about 500,000 troops.