Should America be the world's policeman?

Whenever this question is asked, and it has been asked for nearly 100 years, the answer is usually no. Progressives will say that it suggests American arrogance. Who made America the boss of the world? Many conservatives, especially those with Libertarian leanings, will answer that what other countries do to their neighbors or even to their own people is no concern of ours.

But here's the question that is almost never asked. What's the alternative?

Well, one answer is the United Nations.

That's why the UN was formed after the Second World War - to maintain order and protect human rights.

And that's why there are UN Peace Keepers around the world.

But their record at keeping the peace is abysmal.

To cite just one example, at least 500,000 people were slaughtered in Rwanda in 1994 while UN Peace Keepers stood by.

Furthermore, the United Nations' nearly 200 members seldom agree on anything. And when they do marshal a peace-keeping force, its numbers are always small, they are poorly armed, and highly restricted in their use of the few weapons they do possess. They are much more likely to step aside when faced with aggression than to actively oppose it.

What about dividing the world into spheres of influence? Is that really a good idea? Do we want a world where Russia gets to do what it wants toward democratic neighbors such as Latvia, and Iran dominates its region, and so on? Would such a world lead to peace or to ever more violent competition over the borders of those "spheres."

And, of course, there's an idea that if America leaves the world alone, the world will leave America alone. If only. The Russian revolutionary Leon Trotsky once said that you may not be interested in war but war is interested in you. Great powers don't get to take a vacation and don't get to take themselves off the terrorist target list.

So, we quickly come to an inescapable conclusion: The United States is the World's Policeman because there is no alternative.

And everybody knows it.

But what if Americans don't want the job anymore? What if the cop walked off the beat?

The answer is clear and well grounded both in history and current events: when America retreats, the bad guys advance.

It's not a coincidence that after the US pulled its troops out of Iraq, Islamic State exploded onto the world stage.

And while it's easy to criticize America for its perceived failures in Viet Nam and the first few years in Iraq, what about its obvious successes? After the Second World War ended in 1945, the Soviet Union sought to dominate the world. It failed for one reason: The United States stopped it.

In 1991 we stopped Saddam Hussein and kicked him out of Kuwait. Later, we intervened to save the Balkans.

As I write in my book, America in Retreat, the order the US has provided has not only had enormous security benefits for all the world, it has produced phenomenal economic advantages. Global GDP, just $11 trillion in 1980, doubled by the time the Cold War ended a decade later. By 2012 it reached $72 trillion.

The debate over the value of an American supervised peace, Pax Americana, should have been settled long ago. But history only settles great debates for as long people remember the history. Many college students today could barely identify the Soviet Union, let alone explain how its plans to impose Communism on nation after nation were defeated.

Americans have lived in a relatively orderly world for so long that many have become complacent about maintaining it.

Perhaps that explains why, in recent years, the United States has adopted a foreign policy that neglects to do the things that have made that orderly world possible: Commitments to global security, military forces adequate to meet those commitments, a willingness to intervene in regional crises to protect allies and to confront or deter aggressive regimes.

If the world's leading liberal-democratic nation doesn't assume its role as world policeman, the world's rogues will try to fill the breach. Then the world would be very much like it was in the 1930s, when Western self-doubt, war weariness, economic turmoil, American self-involvement and the rise of ambitious dictatorships combined to produce unprecedented death and mayhem.

Not everyone grows up wanting to be a cop. But no one wants to live in a neighborhood -- or a world -- where there is no cop.

Would you?

I'm Bret Stephens.