Here is part of Andrew Bacevich's answer,which can be summarized as "War is U.S."
in inviting a narrow cost-benefit analysis, the question-as-posed serves to understate the scope of the debacle engineered by the war's architects. The disastrous legacy of the Iraq War extends beyond treasure squandered and lives lost or shattered. Central to that legacy has been Washington's decisive and seemingly irrevocable abandonment of any semblance of self-restraint regarding the use of violence as an instrument of statecraft. With all remaining prudential, normative, and constitutional barriers to the use of force having now been set aside, war has become a normal condition, something that the great majority of Americans accept without complaint. War is U.S.
His complete text and the other three answers are available at the link above.
The other three responders are:
Max Boot, Jeane J. Kirkpatrick Senior Fellow for National Security Studies, Council on Foreign Relations
Michael Ignatieff, Professor, University of Toronto
Michael O'Hanlon, Senior Fellow for Foreign Policy Studies, Brookings Institution