The Pentagon has vowed to preserve the famed nuclear triad despite President Barack Obama’s goal to cut strategic nuclear weapons by as much as a third, avoiding — at least for now — a heated political debate over the future of the nation’s land-, sea- and air-based delivery systems.

At some point, though, the nuclear arsenal may simply become too small to justify the expense of maintaining and replacing the aging bombers, submarines and intercontinental ballistic missiles that make up the triad. Where that point might be, however, is a matter of intense debate.

Indeed, any discussion of altering the triad is a political minefield. The Cold War construct was an accidental doctrine that grew out of intense service rivalries in the 1950s and 1960s. But in the intervening decades, it has become sacrosanct in many quarters, due as much to parochial interests as strategic concerns.

Earlier this year, Defense Secretary Chuck Hagel’s confirmation was dogged by a Global Zero report he co-authored that proposed eliminating the nation’s intercontinental ballistic missiles as part of bilateral negotiations with Russia.

To quell criticism and ultimately win Senate support, Hagel pledged to stand by the triad. Modernizing all three legs, he said, should be a “national priority.”

Just hours after Obama used a June 19 speech in Berlin to call for nuclear reductions, Hagel filled in some blanks and once again vowed to protect the triad.

“As we pursue these reductions, let me emphasize three things — three things that will not change,” Hagel said June 19 at the University of Nebraska. “First, the U.S. will maintain a ready and credible deterrent. Second, we will retain a triad of bombers, ICBMs and ballistic missile submarines. Third, we will make sure that our nuclear weapons remain safe, secure, ready and effective.”

Hagel’s comments might table, temporarily, any official discussion about the future of the triad. On Capitol Hill, Republicans and Democrats alike do not appear to be even considering the possibility of abandoning the three-pronged nuclear deterrent.

“We ought to start off with the premise that we want to keep the triad,“ Senate Armed Services Chairman Carl Levin said last week.

But it is nonetheless a topic that lawmakers and defense officials alike will revisit in the coming years, particularly as the high bills for building new long-range bombers, ballistic missile submarines and ICBMs begin to mount.

Broaching a Dyad

Some current and former military officials have attempted to at least lay the groundwork for that debate in recent years. Adm. Mike Mullen made headlines just days before retiring as the military’s top officer in 2011, when he uttered the word “dyad” in a speech, saying maintaining the triad might become cost prohibitive the smaller the nuclear arsenal gets.

Retired Gen. James E. Cartwright, the former vice chairman of the Joint Chiefs of Staff, helped further fuel the debate when he co-authored the Global Zero report. In an interview last week, Cartwright argued that reducing a leg of the triad could actually benefit national security if Obama’s goal of reducing the arsenal to about 1,000 nuclear warheads becomes a reality.

With a smaller nuclear arsenal, Cartwright argued, there is no room for error. In short, it may be better to fully invest money and manpower in two legs of the triad — building in any necessary redundancies — than spread resources too thinly across all three legs.

“You’re going to have to have a substantially higher degree of assurance that weapons work and that delivery systems will get through,” Cartwright said. “Is there a tipping point somewhere here around the 1,000 number that says that I would have so few missiles or so few bombers or so few submarines that if one of them failed it could be sufficient to actually make my capability not assured?”

Former Rep. Ellen O. Tauscher, a California Democrat who served as undersecretary of State for arms control and international security affairs during Obama’s first term, said the future of the triad is one issue that will be reviewed again and again as the number of weapons decreases.

“There has to be a level of confidence, and for decades we were able to have size provide confidence,” she said. “And when size starts to be less of a crutch, then you’ve got to make tough decisions.”

However, Tauscher does not expect any major changes to the nation’s nuclear posture to happen quickly. The triad, for instance, may continue to be effective until the nation’s strategic warheads dip to the 800 range.

“I’m confident that we are still, on a timeline, a good way away from having to make a decision about our nuclear posture, the size and the breath of the stockpile and the triad,” she said. “But they are looming issues.”

Others believe that keeping the delivery platforms intact could ultimately make getting to lower levels of warheads easier. If supporters of each leg of the triad fear their leg will be the one eliminated, they won’t be on board with further reductions to the arsenal.

“I think you reduce the support for lower numbers [of warheads] if you don’t commit yourself to the triad,” said Morton H. Halperin, a nuclear expert and veteran of the Johnson, Nixon and Clinton administrations.

The political sensitivity is clear. In the immediate aftermath of Obama’s speech, 10 self-described “missile states senators” from Montana, Nebraska, North Dakota, Utah and Wyoming blasted Obama’s proposed reductions and defended the need for ICBMs.

Cartwright said he doesn’t expect to see any resolution to the debate over the size of the nuclear arsenal in general — or the triad in particular — anytime soon. Obama’s speech in Berlin, he said, was simply the first step in an arms-reduction dialogue that will span multiple administrations and Congresses.

“This is going to cross into the next administration, which I think is probably a good thing,” Cartwright said. “He has allowed a process by which it won’t just be an edict.”