In yesterday’s “Twitter Town Hall,” President Barack Obama demonstrated yet again his failure to understand the size of the defense budget—as well as his desire to slash it in order to fund domestic spending. Here’s how the President described it:

[T]he nice thing about the defense budget is it’s so big, it’s so huge, that a 1 percent reduction is the equivalent of the education budget. Not—I’m exaggerating, but it’s so big that you can make relatively modest changes to defense that end up giving you a lot of head room to fund things like basic research or student loans or things like that.

Think again, Mr. President. Contrary to his wildly exaggerated statement, a 1 percent reduction to the Pentagon’s proposed fiscal year 2012 base budget would be $5.5 billion—or 7 percent of the Department of Education’s proposed FY 2012 budget.

The President’s accounting failures aside, there’s an even bigger problem at work. Obama is of the belief that, for starters, $400 billion can be cut from the defense budget over the next 10 years without putting the military at risk. That’s in addition to the approximately $400 billion already cut by the Administration during the previous two years. In turn, he would take those dollars and apply them to pay for his pet projects at home.

The President is proposing those cuts irrespective of the military’s needs.

Outgoing Secretary of Defense Robert Gates stated that ill-conceived cuts to defense spending could increase America’s vulnerability in a “complex and unpredictable security environment” and that “the ultimate guarantee against the success of aggressors, dictators, and terrorists in the 21st century, as in the 20th, is hard power—the size, strength, and global reach of the United States military.”

But with the President’s proposed cuts, America’s base defense budget would be at its lowest point in more than 60 years (as a percentage of America’s GDP). Meanwhile, the threats Gates spoke of continue to materialize, while challenges remain in Iraq, Afghanistan, Libya, and throughout the Middle East.

And then there’s the state of U.S. forces. Secretary Gates and the Quadrennial Defense Review Independent Panel have agreed that the U.S. went on a “procurement holiday” in the 1990s. Air Force Chief of Staff General Norton Schwartz has stated that the present fleet of 187 F–22 fighters creates a high risk for the U.S. military in meeting its operational demands. The U.S. Navy has the fewest number of ships since America’s entrance into World War I. And yet the President sees fit to slash defense?

Contrary to Obama’s belief, the defense budget is not an ATM from which he can pull cash to pay for other projects. And he certainly can’t do it without causing further damage to U.S. military readiness. The Constitution demands that the U.S. government provide for the common defense. That’s a fact the President should keep in mind as he looks for ways to increase domestic spending amid a debt crisis.