The study published by Brown University this week focused on the wars in Iraq, Afghanistan and counter-terrorism campaigns in Pakistan and Yemen, which came in the wake of the 9/11 attacks on the United States.
The authors argued that governments almost always go to war underestimating the potential duration and costs of a conflict while overestimating "the political objectives that can be accomplished by the use of brute force."
The study said "an extremely conservative estimate" of the casualty toll was about 225,000 people killed and 365,000 wounded in the wars so far.
The number of soldiers killed comes to 31,741, including about 6,000 Americans, 1,200 allied troops, 9,900 Iraqis, 8,800 Afghans, 3,500 Pakistanis as well as 2,300 US private security contractors, it said.
The civilian toll was much higher, with an estimated 172,000 dead, including about 125,000 Iraqis, 35,000 Pakistanis and 12,000 Afghans, it said.
The study acknowledged that estimating the number of dead was difficult, particularly the toll for insurgents, putting the number at between 20,000 to 51,000 insurgents killed.
The report found that 168 reporters and 266 humanitarian workers were among the dead since the United States launched its "war on terror" after 9/11.
The wars also have triggered a massive flow of refugees and displaced persons, with more than 7.8 million displaced, mostly in Iraq and Afghanistan, it said.
The study estimated the financial cost of the wars at a minimum of $3.7 trillion and up to $4.4 trillion, which represents about a quarter of the country's current debt, accoridng to AFP.