We are rightly mourning the horrific killings in Newtown, Connecticut’s Sandy Hook Elementary School and discussing the threats posed by semiautomatic rifles.

Yet, on another front, the United States is moving ahead with plans to spend hundreds of billions of dollars on nuclear weapons — an even more destructive force — with no serious public discussion.

Twenty years from now, how many nuclear warheads on strategic submarines will the U.S. need?

That’s not an abstract question. The country is engaged in a costly, ambitious modernization of its nuclear weapons complex and development of a new generation of delivery systems — new strategic submarines, bombers and intercontinental ballistic missiles that will be operating more than 50 years from now.

Start with the navy’s plan for 12 new SSBN-X strategic submarines to replace the 14 Ohio-class subs now in service.

A Congressional Research Service report on the program, released Dec. 10, asks whether the navy can stay within the cost targets for their procurement ($4.9 billion each) and whether each sub should carry 16 or 20 missiles.

But shouldn’t the questions be more basic, such as who is the enemy and how many subs would be needed to deter that enemy?

There will be at least four or five warheads on each of the 16 ICBMs carried on each of the new subs. Their destructive power will be eight to more than 20 times that of the atomic bomb that all but destroyed Hiroshima in 1945.

That bomb killed 45,000 men, women and children instantly. Another 19,000 died during the next four months, according to a 1946 study by the Manhattan Engineer District, which built the bomb. The majority of those killed were civilians, though Hiroshima was picked because planners saw it as a military target with army barracks and defense factories.

But the bomb — its 12.5 kiloton explosive power [equal to 12,500 tons of TNT], its heat effects and radiation — went well beyond those military targets. Today’s nuclear weapons are 100 kilotons and above.

It’s agreed that nuclear weapons don’t deter terrorist groups. And if history is any guide, the more the U.S. and other nuclear-armed countries modernize their weapons, the more tempting it is for other countries to want nuclear arsenals.

So how many warheads does the U.S. need over the next 40 years to deter others?

That’s a multibillion-dollar question, and among those President Barack Obama and his new national security team will have to wrestle with as they try to tighten defense spending.

I have written before that it’s time to get a rational, long-range nuclear strategy because the cost of replacing the nation’s three nuclear delivery systems will top $100 billion and require another $300 billion over the next 10 years to keep them operational.

The Cold War created a mindless U.S.-Soviet Union nuclear arms race in which both sides forgot the power of the weapons they were building and believed that whoever had the largest number was the strongest. Numbers on both sides went close to 20,000 bombs and warheads. It took just two to end World War II in the Pacific, and the threat of using one ended the 1962 Cuban missile crisis.

Since 1991, when the Iron Curtain fell, both the U.S. and Russia have sharply reduced not just their overall stockpiles but also their deployed weapons.

According to a study released recently by Hans Kristensen of the Federation of American Scientists, both sides are down to roughly 4,500 strategic warheads and bombs apiece, and by 2018 will have just 1,550 operationally deployed as required by the New Strategic Arms Reduction Treaty, which took effect Feb. 5, 2011. Ironically, after signing the treaty, the two countries began modernizing their nuclear forces.

By the end of the decade, the deployed U.S. force may be 400 single-warhead, land-based ICBMs; 240 submarine-launched ballistic missiles with three to five warheads each; and 60 strategic bombers, which each count for only one warhead though they carry more than one bomb. Beyond that, there are to be some 1,600 stockpiled warheads or bombs, Kristensen says.

Why do we need that size of a nuclear arsenal for the next 50 years?

Why 12 and not 10 subs, for example? Under construction plans, the navy will go down to 10 operational boats between 2029 and 2041, as old Ohio-class submarines are retired before new ones are finished, according to the CRS study.

What new threat requiring another 90 sub-launched warheads will be arising after 2041?

A new Presidential Policy Directive is due to be presented to the military shortly, a paper in which Obama will set nuclear force planning for the rest of his administration.

In his April 2009 speech in Prague, the president said he wanted to “put an end to Cold War thinking … reduce the role of nuclear weapons in our national security strategy and urge others to do the same.”

The country should be watching to see how he implements that promise as much as they are waiting to see how he follows up on his pledge in Newtown to use “whatever power this office holds to engage my fellow citizens … in an effort aimed at preventing more tragedies like this.”

Walter Pincus reports on intelligence, defense and foreign policy for The Washington Post and writes the Fine Print column.