The Pentagon has a dream that it won't give up: blasting any target on the planet with a submarine's missile. Nothing seems to stop it, not even years of protest that the project could accidentally spark a nuclear war. But now, the Pentagon swears, it's figured out how to launch the missiles without triggering any inadvertent Armageddon, and is pushing the concept in its new budget.

One problem: No one at the Pentagon can seem to agree on what the latest iteration of this so-called "Conventional Prompt Global Strike" concept really is.

Here's the basic problem with the plan. A ballistic missile fired with a conventional warhead flies in the same trajectory as a ballistic missile fired with a nuclear warhead. Seeing any such missile in the air could prompt a panic in Moscow, Beijing or another nuclear-armed capitol. So while Washington thinks it's striking a terrorist training camp or an enemy weapons silo, it might prompt someone else to let loose the world's most dangerous weapons.

But now the Pentagon's budget, unveiled Thursday, returns to the much-derided concept, calling for a "design of a conventional prompt strike option for submarines." Unveiling the decision, Defense Secretary Leon Panetta beamed, "the Navy will invest in a design that will allow new Virginia class submarines to be modified to carry more cruise missiles and develop an undersea conventional prompt strike option."

That seemed to suggest that Panetta expected the sub strikes to use cruise missiles instead of ballistic missiles. That would probably take care of the problem of nuclear confusion, since cruise and ballistic missiles fly across the sky differently. But it might not be a truly prompt or global strike option, since cruise missiles don't have the range or speed that ballistic missiles do. Plus, it would take time to get a sub into position to fire.

But Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, muddled the issue by suggesting the Pentagon actually had a different, tested, technological fix in mind. Asked during Thursday's budget briefing how to avoid confusing the Russians and the Chinese with Conventional Prompt Global Strike, Dempsey said that "technology" had changed "the trajectory that would be required to deliver" a conventionally-armed ballistic missile. So had "the speed with which these systems can move. And therefore, you can lower the trajectory, and therefore avoid the confusion you're talking about in terms of it being mistaken for an ICBM [intercontinental ballistic missile] with a nuclear warhead."

That's a dubious proposition. While there are design alternatives in the works, none of them are anywhere close to weapons-ready. These are still very much research experiments. No sub will get one for years and years, if ever.

Most of the experiments center around a new type of warhead for the ballistic missile: a hypersonic glider. Unlike a old-school warhead – which pretty much goes into space on top of the missile, and then comes crashing straight down to its target – a glider drops into the atmosphere and then flies parallel to the Earth. If a standard warhead has an upside-down-U-shaped trajectory, the hypersonic glider's looks like a backwards-L. That's called "boost-glide" in missile jargon.

Or at least that's the theory. The tech, despite Dempsey's assurances, isn't there yet in practice. A Darpa initiative to create a Mach-20 glider was kind of a #fail. (Like, actually. Darpa live-tweeted its disappointing test this August.)

The Army, using a different design – one that looked more like a typical missile, not the slice of deadly deep dish pizza that Darpa developed – succeeded. But its Mach-8 glider faced an easier test than Darpa's did. In November, the Advanced Hypersonic Weapon boosted off from Hawaii and descended on its target in the Kwajalein Atoll in the South Pacific, 2,400 miles away. That's about 60 percent as far as the Darpa glider was trying to go, and at 40 percent of the speed. Still, it's something to score on conventional prompt global strike's ledger.

Only it may not be something the subs can capitalize on. The missile tubes on U.S. subs are too small to launch the Advanced Hypersonic Weapon, let alone Darpa's Mach-20 glider. The agency did have plans for a hypersonic glider that could fit inside a sub's launch tube. But that "ArcLight" program was officially canceled before it could, you should excuse the expression, get off of the ground.

Nevertheless, it seems that some in the Pentagon are, in fact, talking about putting the gliders on subs. "The conventional prompt strike concept from a submarine could be an intermediate range boost glide capability," says Lt. Col. Melinda Morgan, a Pentagon spokeswoman. Panetta, Morgan says, didn't mean that the subs will use cruise missiles for Conventional Prompt Global Strike; the extra cruise missiles will be used for other missions.

Perhaps the Pentagon is looking to revive ArcLight. Perhaps the Pentagon is hinging its hopes for a sub-based long range strike on a technology family that's passed one (relatively) easy test. Or perhaps Morgan and Dempsey are wrong and the idea really is to go with Panetta's cruise missiles.

But the moment, confusion reigns. And confusion has characterized the project almost since the Obama administration resurrected it from the Bush administration's failed plans. Last year, Air Force generals and civilians repeatedly contradicted each other on whether gliders, ballistic missiles or some combination of the two would be the centerpiece of the global-strike project.

And there's another, deeper problem. Nuclear-armed nations like Russia might not care if a missile – ballistic, cruise or hypersonic – carries a conventional warhead. After all, the point of the strike capability is to let the U.S. hit anywhere on earth in mere hours or less.

"The Russians don't care – they're worried about it even if it's conventional," explains Tom Collina of the Arms Control Association. "They think it's a strategic conventional capability,and it's a complete mismatch in the discussion."

Even if the Pentagon figures out what kind of sub-launched strike it's really talking about, there's no technological fix for that.