Traditionally, military planners have operated under a worst-case scenario: i.e., what do we need to have in place to respond if nothing goes as planned? The Obama administration and Congress appear to be operating under a best-case scenario: i.e., what is the minimum force we can field on the assumption that nothing will go terribly wrong?

Thus the new defense budget, being unveiled today, which cuts the army’s active-duty force size to the smallest level since before World War II–just 440,000 to 450,000 soldiers. That’s down from a wartime high of 570,000, although even that figure was painfully inadequate to allow the U.S. to respond to two unforeseen wars in Afghanistan and Iraq.

As critics of the Bush administration–including Senator Barack Obama–were once fond of pointing out, Bush never sent enough troops to stabilize Iraq until 2007 and that commitment was only made possible by keeping a ludicrously small force in Afghanistan, once known as the “necessary” war. The failure to send more troops early on allowed the Taliban to rebound from near-defeat in 2001 and allowed various insurgent groups to sprout all over Iraq.

So if 570,000 troops were not enough to handle such relatively weak foes as al-Qaeda in Iraq and the Taliban, how on earth would 440,000 troops be able to handle more robust contingencies–unlikely but not impossible–such as simultaneous wars with Iran and North Korea and a stabilization mission in, say, Yemen? The answer is that they couldn’t.

Actually the situation is even worse than the news would have you believe. Because the army’s plan to cut down to 440,000 to 450,000 is premised on the assumption that Congress will continue to provide relief from half a trillion dollars in sequestration cuts. But the budget deal reached by Rep. Paul Ryan and Sen. Patty Murray only provides sequestration relief in 2014 and 2015; unless Congress is willing to turn off sequestration in future years, the army will have to go even lower in end-strength.

Moreover, the defense budget includes modest cuts in personnel spending–spending on pay, pensions, and health care–which are long overdue but which are likely to be blocked by Congress, as was the case with a recent attempt to cut cost-of-living adjustments for military retirees by a measly one percent. Unless Congress goes along with cuts to personnel costs, which now constitute half of the defense budget, other parts of the budget–including, no doubt, the army’s end-strength–will have to endure further scaling back.

That is a responsible decline in military strength only if you assume that we will never fight another major land war, or engage in simultaneous stabilization and counterinsurgency operations. And that, in turn, is a tenable assumption only if you assume that the laws of history have been repealed and a new era is dawning in which the U.S. will be able to protect all of its vital interests through drone strikes and commando raids. We all hope that’s the case but, as the saying has it, hope isn’t a strategy. Except, it seems, in Washington defense circles today.

If history teaches anything, it is that the era of land wars is not over and that we will pay a heavy price in the future for our unpreparedness–as we have paid in blood at the beginning of every major war in American history. Our failure to learn from history is stunning and (from a historian’s standpoint) disheartening but not, alas, terribly surprising: Throughout history, supposedly enlightened elites have been able to convince themselves that the era of conflict is over and a new age is dawning. The fact that they have always been wrong before does not, somehow, lead them to question those assumptions in the present day, because this is such a convenient belief to have.

Today, for both Republicans and Democrats, the president and Congress, these hope-based assumptions about defense spending allow them to put off the truly difficult decisions about cutting entitlement spending. But at what cost? If history is any guide, the cost of unpreparedness will be steep and will be borne by future generations of American troops.