Professor Colin Dueck argues in Hard Line that dedication to a strong national defense is the most enduring theme of conservative and Republican foreign policy since World War II. In spite of "apparent oscillations between internationalism and isolationism," conservatives have demonstrated "a hawkish and intense American nationalism … committed to building strong national defenses, determined to maintain a free hand for the United States internationally."

There's a reason for this. Though skeptical of government's domestic ambitions, conservatives instinctively understand it has a special role — spelled out in the preamble to the Constitution — "to provide for the common defense." According to a recent poll in 10 battleground congressional districts, 60 percent of likely Republican voters won't accept any cuts to defense and homeland security.

Even fiscal hawks like Rep. Paul D. Ryan, Wisconsin Republican, understand government's unique role in defense. Mr. Ryan agrees there's a lot of waste to cut from the federal budget. But, he argues, "Let's not do so at the expense of our fundamental, primary function of our federal government, which is to secure our national defense … you can't take money from defense to plow it into all this domestic spending." Likely presidential contenders Newt Gingrich and Mitt Romney also fully embrace the Reagan vision of "peace through strength."

So don't assume all conservatives will surrender to "budget hawk"-inspired isolationism.

However, conservatives strongly believe in accountability. They understand we can find savings in logistics and personnel management and compensation reform. Reforming performance logistics alone could save as much as $32 billion a year.

Unfortunately, there are not enough sound savings within defense to fill the defense budget hole. According to the military services, their modernization budgets fall about $50 billion per year short of what's needed to develop essential next-generation equipment. The Congressional Research Service estimates the requirement gap is $30 billion to $40 billion for the Army alone. Naval shipbuilding plans and Air Force aviation plans are similarly underfunded.

The case for replenishing the defense budget is sound, but conservatives must do a better job of explaining it to the American people.

First, they must explain why defense is different — why its budget shouldn't be on the chopping block along with domestic programs.

One reason is because the Constitution says so. George Washington pushed for a constitutional convention because he saw firsthand how the Articles of Confederation failed the nation in war. We needed better coordination and funding mechanisms. Providing for a better defense became a major driving force for approving the new Constitution and a founding principle of the nation.

Second, conservatives must explain what's happening to the defense budget. What drives our fiscal crisis is not defense expenditures but entitlement spending. It, along with interest on the debt, accounts for more than 65 percent of the federal budget; defense spending claims less than one-fifth. As a percentage of gross domestic product, or GDP, defense spending is near historic lows.

If you think we can afford to spend less today because we are in an age of peace, think again. The military is still fighting in Iraq and Afghanistan, and our nation faces growing threats from terrorists and rogue nations.

Armies and navies not only cost a lot to maintain; they need to be funded and modernized over long periods. We must look ahead many years to determine what forces we need to reduce the risks. We work backward from there to determine how much such forces will cost, with infrastructure and modernization costs built in requiring budget support over many years.

Enemies of long-term military strength include: (1) Weapons and program cancellations, often after billions have been invested in development, and (2) budgetary ups and downs, introducing tremendous instability into long-range military planning, which causes sharp and wasteful increases in weapons costs.

The fact is that having fought two wars for a period of years, our military equipment and forces are wearing out. The average age of our tactical aircraft is 20, bombers nearly 30, and tankers about 45 years old. And yet investment in new weapons has stalled or been cut back.

America needs the best military force in the world. We are a global power with global interests, and the threats are real. Imagine the casualties if we lacked a missile-defense system to knock down a future barrage of Iranian nuclear missiles fired at New York, or the economic consequences if our naval and ground expeditionary forces were insufficient to stop a run on Middle East oil supplies. Consider the global instability if we have "no go" areas in Asia and Europe, for example, because our military becomes too weak to predict victory.

These are the hard realities of national security. Conservatives know we must fix our debt problem. They also know it can't be done safely if we fail to ensure the nation's security.

Kim R. Holmes, a former assistant secretary of state, is a vice president at the Heritage Foundation