For most of us, it’s the season of sun, sand and backyard barbecues. But the U.S. Conference of Mayors seems to think it’s Christmas.

And all because of one key sentence in President Obama’s recent address on withdrawing forces from Afghanistan more quickly: “America, it is time to focus on nation-building here at home.” The mayors anticipate a windfall that can be spent on their pet projects. The prospect of a “peace dividend” is creating wish lists from Albany to Albuquerque.

As Los Angeles Mayor Antonio Villaraigosa put it, “We need to use the billions of dollars we are currently spending in Afghanistan to rebuild our domestic economy.”

What we need to do first, actually, is something quite different. Something very important. We need to ensure that we’ll continue to have a world-class military. That means we need to invest a good portion of that “peace dividend” in the very reason we have peace in the first place: our over-worked armed forces.

Let’s not delude ourselves. The fact is, we’ve been living for a while now off past military build-ups. But things wear out. Equipment breaks down. And it can be patched up only so many times before it’s unable to do the job it’s meant to do. It’s time to make airplanes that are younger than their pilots, and ships that are younger than their commanders.

Otherwise, we risk repeating cycles of the past. When we cut too deeply, as we did during President Carter’s time in office, we degrade our ability to defend ourselves. We end up with humiliations such as the Iranian hostage crisis, and watch as emboldened enemies reassert themselves abroad. This necessitates a rapid build-up -- which, fortunately, we got during the Reagan years. Eventually, though, voices call for a drawdown in military spending. Domestic priorities beckon. Too much is cut -- and the cycle begins anew.

We have to remember that at any given time we’re enjoying the fruits of past military build-ups. Today’s security was purchased with yesterday’s dollars. And if we continue to invest too little today, we’ll be paying for it (so to speak) with tomorrow’s underfunded military.

 “The Army and Marine Corps in particular now need to reinvest in their people, in tracked and wheeled vehicles, and in helicopters -- after being employed at wartime-usage rates for a decade and in harsh conditions,” Heritage Foundation defense expert Mackenzie Eaglen recently told The New York Times.

But under defense budget plans being advanced by the Obama administration, such necessary spending won’t happen. In fact, officials expect the Pentagon to cut some $400 billion. That means fewer destroyers, carriers and submarines for the Navy -- and less maintenance on ships. It means the Army and Marine Corps won’t be able to upgrade as many Humvees. And the Air Force can forget about the next-generation aircraft to help keep the skies safe. Anyway, it will be busy enough trying to make its existing aircraft last longer.

It’s easy to forget, too, that defense spending makes up a very small portion of the federal budget. And it has increased at a much lower rate than domestic spending in recent years. Even while fighting two wars, the core defense budget has risen about $220 billion since 2001, about a tenth of what the government devotes each year to programs such as Social Security, Medicare and Medicaid.

This isn’t to suggest that cuts in military spending are impossible. There’s waste in every area of government, and defense is no exception. But we have to be smart about where and what we cut in what is, after all, one of the federal government’s foremost responsibilities. A knife is needed, not an axe. We can’t keep the peace if our military is in pieces.