By John Solomon and Buck Sexton

President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE took a little time during a policy-rich interview in the Oval Office to give his take on the biggest mistake in American history.

Was it the Civil War? Nah. The failure to stop Sept. 11? Nope. How about Pearl Harbor? Not even close.

“The worst single mistake ever made in the history of our country: going into the Middle East, by President Bush,” the president said during an exclusive interview with Hill.TV. “Obama may have gotten (U.S. soldiers) out wrong, but going in is,to me, the biggest single mistake made in the history of our country.”

So why was it so catastrophic?

“Because we spent $7 trillion in the Middle East. Now if you wanna fix a window some place they say, 'oh gee, let’s not do it.' Seven trillion, and millions of lives — you know, ‘cause I like to count both sides. Millions of lives,” the president explained.

“To me, it's the worst single mistake made in the history of our country. Civil war you can understand. Civil war, civil war. That’s different. For us to have gone into the Middle East, and that was just, that was a bad day for this country, I will tell you.”

Different estimates exist on the costs of the U.S. wars in Iraq and Afghanistan, which Trump has frequently criticized.

A recent estimate by Brown University put the cost, as of September 2017, and $5.6 trillion, a total that includes costs associated with the two U.S. wars, military action related to Pakistan and Syria, homeland security expenses and health-care costs for veterans of the wars.

The Pentagon estimates total expenditures related to the wars in Iraq, Afghanistan and Syria between 2001 and 2018 are $1.52 trillion.