The study ranked the United States 97th out of 140 countries according to how peaceful they were domestically and how they interacted with the outside world.
The United States slipped from 96th last year, but was still ahead of foe Iran which ranked 105th. It, however, lagged Belarus, Cuba, South Korea, Chile, Libya and others which were listed as more peaceful.
Iraq, which the United States invaded in 2003, leading to the toppling of Saddam Hussein, ranked lowest on the index. Afghanistan was also in the bottom five, along with Sudan, Somalia and Israel.
Russia ranked 131stCommenting on the US ranking, State Department spokesman Sean McCormack said to realize a more peaceful and prosperous world, "often times, you have to do difficult things and a lot of times, people don't agree with them. They don't like them."
"A lot of times you fall down in these lists but at the end of the day it is in defense of democracy and the way of life we have enjoyed over the past several decades," he added.
The index looks at 24 indicators of external and internal measures of peace, including UN deployments overseas and levels of violent crime, respect for human rights, the number of soldiers killed overseas and arms sales.
The Group of Eight major economic powers were a mixed bag. Japan ranked fifth, Canada 11th, Germany 14th, Italy 28th, France 36th and Britain 49th. Russia was near the bottom at 131st, the only one in the group below the United States.
Supporters of the index urged policymakers to focus more on education, wealth, and well-functioning government and pointed to the role of business in creating more stability.