In evaluating Defense Secretary Chuck Hagel’s proposed downsizing of the U.S. military, particularly the active-duty Army, it’s worth remembering the words of a predecessor. “

As you know, you go to war with the Army you have,” Donald Rumsfeld said in 2004. “They’re not the Army you might want or wish to have at a later time.”

This was even as the Iraq war, after a dominant race to Baghdad, was blowing up in Rumsfeld’s face, in part because his goal of a smaller-footprint, agile and nimble fighting force left out some crucial and unanticipated details, like what to do about ground and enemy troops conquered en route to what appeared to be the ultimate goal.

Today, Hagel, once a sharp Bush administration critic, also wants a more nimble fighting force that relies less on materiel and manpower, each of which comes at a cost to the federal budget. Hagel would be wise to make sure his team’s cuts do not go so deep as to hinder future presidents in confronting the world’s dangers.

The reality, as Hagel puts it, would leave the U.S. with its smallest active-duty Army since before World War II. Going away would be the entire fleet of A-10 “warthog” anti-tank planes, other weapons systems and some benefits that supplement service members’ pay. Also gone would be the thought of a U.S. military capable of fighting two major conflicts simultaneously. Adm. Michael Mullen, former chair of the Joint Chiefs of Staff, called America’s debt “the most significant threat to our national security,” and the need to include the military in each round of budget cuts is one reason. But even as we acknowledge that inevitability, we’d love to see Hagel’s boss show some of the same enthusiasm for tightening other line items in his budget, too.

It’s simple, even logical, to think that with Iraq behind us and Afghanistan done soon that the time is right for deep cuts. Yet Iraq and Afghanistan were protracted conflicts, far longer and more costly than anyone could have predicted. The world’s challenges are similarly difficult to forecast, yet the U.S. will remain, for decades to come, atop a very short list of superpowers to call in a crisis. That burden has and will fall disproportionately on an all-volunteer military, the world’s finest — and most expensive — fighting force. With known trouble spots like Syria, Iran and North Korea and any number of stable-today-crisis-tomorrow regions, Hagel and Obama would be wise to expect the unexpected and have a place to turn when the world needs it.