Twice in modern U.S. history, irresponsible foreign policies have invited huge and costly disasters.

The isolationism of the 1920s and 30s yielded World War II. And Jimmy Carter's 1970s experiment of putting a happy face on American power (much like we see happening today) wound up with crises in Iran and Afghanistan.

U.S. disengagement from international affairs after the First World War convinced our enemies that we would not resist aggression abroad. That misperception exacted a terrible price among friend and foe alike. Postwar U.S. leaders resolved never again to let overseas threats become so great that defeating them would cost more than preventing them.

In the 1970s, Mr. Carter took a "vacation" from this hard-learned lesson. He apologized for America's "inordinate fear of communism," cut vital defense programs and withdrew support for allies like the Shah of Iran. It was disastrous. The Soviets invaded Afghanistan. Iranian fundamentalists revolted, taking Americans hostage. Mr. Carter exploited a political and economic crisis to usher in a new foreign policy paradigm - ostensibly to correct past mistakes (like Vietnam), but in reality retreating from the post-World War II model of American power.

While Ronald Reagan restored the peace-through-strength model, the Obama administration has adopted an apologetic, soft-power approach similar to that of the Carter administration. Unless we reverse course, well reap the same Carteresque results: more instability, more attacks against the U.S. and more loss of life and treasure.

Signs of instability amid a growing perception of American weakness are everywhere. Allies and friends are beginning to hedge their bets, distancing themselves from us:

• Turkey, a NATO ally, joins Iran, Venezuela and Brazil to weaken sanctions on Iran.

• A British candidate for prime minister calls the "special relationship" with America an anachronism.

• Nine months after President Obama cancels our missile defense deal with Poland, Warsaw invites Russia to join the European Union Eastern Partnerships "group of friends."

• Fearful that U.S. support is slipping, Israel increasingly takes defense matters into its own hands.

Nonaligned nations, skittish about Americas receding projection of power, are joining the exodus from American influence. Ukraines parliament recently adopted a law effectively preventing it from ever joining NATO - something unthinkable a few years ago, but sure to please Russia today.

Yet another sign: Rebukes and aggression by our adversaries are rising.

• China angrily lectures a delegation led by Secretary of State Hillary Rodham Clinton that America is a "hegemonic" power, and snubs Secretary of Defense Robert M. Gates request to visit Beijing for talks on military affairs.

• Iran, seeing U.S. reticence to back Israel after the flotilla confrontation, threatens to send its own ship to test the Gaza blockade.

• North Korea attacks our ally South Korea with apparent impunity and now threatens war because Seoul has taken the incident to the U.N. Security Council.

These are not the actions of nations confident in or respectful of American power and leadership.

But the brightest warning signal that we are entering the Carter-era redux is the revival of the notion in Washington that U.S. military strength is not all that important. We see it when we "follow the money."

One of the few federal agencies from which this administration demands "fiscal responsibility" is the Department of Defense. White House spending projections would increase budget outlays for the General Services Administration (by 22 percent), the Treasury Department (35 percent), and foreign aid (18 percent) over the next two years, but cut the defense budget (by 5.5 percent).

Mr. Gates calls for cutting off a "gusher" of defense spending. But the only thing "gushing" here is wasteful domestic spending. The administration may promise future cuts of non-defense spending, but it already has shown through concrete actions where its real priorities are.

Certainly, we should cut wasteful Pentagon spending (Heritage analysts estimate that we could save $32 billion a year in military logistics alone). But that money needs to stay in the defense budget to purchase new weapons and equipment - not to offset hikes in foreign aid or domestic programs.

The real culprits driving our exploding national debt are social entitlements and other domestic spending. Defense spending is one of the few budget areas where spending is actually declining as a percentage of the gross domestic product. Indeed, rising entitlement and domestic spending threatens to crowd out national defense spending more and more.

Americans are rightly outraged because government spending is 1. Too high, and, 2. Misallocated. But national defense is not optional. The U.S. Constitution establishes it as the central responsibility of the federal government.

There is a huge difference between spending taxpayer money to get clunkers off the road and building a missile defense system that protects Americans from nuclear attack. The first is unnecessary and of dubious constitutionality. The second is absolutely necessary and mandated by the Constitution.

We need a strong military and strong American leadership today for the same reasons we needed them in the past: to prevent wars from happening in the first place, and to fight and win them should we fail in that noble endeavor.

Above all, we should remember that declining military strength goes hand in hand with weakness and timidity, which inevitably demoralize allies, repel potential friends, and embolden enemies.

The country paid dearly when Mr. Carter forgot this maxim. We cannot afford to relearn that lesson today.

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