The US government has wasted $30bn (£18bn) in contracts in Afghanistan and Iraq over the last decade, according to a bi-partisan spending commission.
The commission on wartime contracting blamed an over-reliance on contractors, poor planning and fraud for the waste.
It had evidence of lax accountability and inadequate competition, it said.
Writing in the Washington Post, the report's authors warn that investments in the two countries could be wasted even after US involvement there ends.
Among the examples cited was a $40m prison for Iraq that the country did not want and was never completed.
US-funded projects in those two countries also risk going to waste because host governments are unable or unwilling to sustain them.
In one case, $300m was poured into a sophisticated power plant in Kabul which the Afghan government will not be able to run, and a programme worth $11.4bn of facilities for the Afghan national security forces is likely to be unsustainable.
The commission's report is due to be published on Wednesday but its authors, who include former government officials and lawmakers, have already published an opinion piece in the Washington Post, blaming both the government and contractors for the waste.
The article makes the obvious recommendation to be more rigorous when deciding whether a project should be contracted out but it also recommends cancelling or modifying projects that are not sustainable.
Beyond just a disservice to taxpayers, the report says that the waste fosters corruption in host countries and diminishes the standing and influence of the US.