It’s no surprise when defense secretaries sound warnings about the perils of military spending cuts. After all, they have a duty to serve as advocates for sufficient Pentagon funding.

But it still was unsettling last week to hear former Defense Secretary Robert Gates say that he was “double-crossed” by President Barack Obama on White House assurances of military appropriations.

Mr. Gates, who served as defense secretary from 2006-2011 under both Presidents George W. Bush and Obama, has earned a reputation for providing well-informed — and nonpartisan — perspectives on military and international issues. So the concerns he raises about the decline in U.S. military capability should sound a lasting alarm.

And as Americans ponder their choices of who will replace our current commander in chief just 9½ months from now, candidates for all federal elective offices should address the proper, practical responsibilities of America’s armed forces — and the resources required to carry them out.

Former Secretary Gates, during a Fox News special titled “Rising Threats — Shrinking Military,” which first aired last Friday night, said that he went along with large reductions in defense spending based on President Obama’s assurances that there would not be more “significant changes” in the military budget after that.

However, when asked if the president kept his word on that pledge, Mr. Gates replied: “Well, I think that began to fray. ‘Fray’ may be too gentle a word.”

Certainly the president’s demand for hundreds of billions in additional defense spending cuts wasn’t a gentle adjustment in the Pentagon’s plans.

As Mr. Gates bluntly put it: “I guess I’d have to say I felt double-crossed. After all those years in Washington, I was naive.”

Meanwhile, any president, including Mr. Obama, who believes that steep reductions in U.S. military strength don’t embolden our enemies is also naive.

Mr. Gates said he urged President Obama to soften the cost-cutting blow — to no apparent avail. The ex-defense chief explained:

“I think he acknowledged that what I was pitching at a minimum was, ‘The world doesn’t seem to be getting better. Before you head down a path of deep cuts in defense, why don’t you take it kind of slow.’ You know it was one of those things where I lost the argument.”

But the high-stakes argument over America’s role in the world — including our military’s role — persists.

No, we can’t afford to play world cop in every conflict across the globe. The costs are simply too great in both blood and treasure — especially with the record $19 trillion national debt climbing ever upward.

Nor, however, can we afford to rashly retreat from the long-time U.S. leadership position in global affairs — a crucial task that demands at least the potential of American military intervention. Abandoning that function would impose potentially disastrous costs, too.

Unfortunately, though, the current political campaign season has delivered a surplus of frivolous controversies and a severe shortage of serious foreign and defense policy debates. From both parties, the discussion of defense issues has been superficial at best, irresponsible at worst.

And in this election year, it’s not enough for White House and congressional candidates to decry — or defend — the Obama administration’s steep reductions in U.S. military capability.

They should tell voters what missions they would assign to our armed forces — and how they would pay for them.