Not long after Afghanistan goes off its meds — American and other troops, oodles of aid, civilian technicians, and so much cash that airplanes have to strain to smuggle it out of the country — it will revert to Afghanistan. The Taliban will come down from the mountains and from Pakistan and either make a deal with the government or reassume control. This is not only what will happen in the future. It is what has happened in the past.

From the start, America’s huge investment in Afghanistan has been a mistake. It was always necessary, not to mention just plain right, to go after Osama bin Laden and kill every last member of al-Qaeda. That job has mostly been done. But the rest — the routing of the Taliban and the building of a democratic state — is beyond America’s reach. The troops — most of them — should come home.

President Obama has always shown a commendable lack of enthusiasm for the Afghanistan war. His 2009 decision to commit an additional 30,000 troops to the effort — the so-called West Point surge — was an obvious split-the-difference calculation, fewer troops than the military wanted, many more than war critics thought were warranted. Now, Obama must decide how many of America’s approximately 100,000 troops should remain. As few as possible would be the wise decision.

The trouble with recommending such a course is that it conforms to the foreign policy views of almost all Republican presidential candidates. Their position regarding Afghanistan is, however, just a piece of their wholesale embrace of Herbert Hoover Republicanism. They would turn the country inward — what John McCain and Lindsey Graham characterize as isolationism — while also adopting Hoover’s disastrous economic policy. The historical ignorance so obvious in our youth is an appropriate homage to their GOP elders. Not satisfied with a recession, they would cut government spending and bring on a depression.

The Republican response to both foreign and domestic problems somehow fits what is beginning to look like the 1930s all over again. Back then, a severe worldwide depression encouraged the rise of fascist and communist movements and turned nations inward. The situation is now not as dire, but when outgoing Defense Secretary Robert Gates complained about NATO’s reluctance to actually do something in Libya, he was talking about governments that are severely pinched — some of them, such as Greece, Portugal and Spain, teetering on the cusp of bankruptcy. The same inverted demographic dilemma that confronts the United States — too few young to support too many old — confronts the rest of the industrialized world. China, with its one-child policy, is soon to learn what I mean.

The return of the ’30s is to be both lamented and feared — the decade led to a world war, after all — but it also has to be acknowledged. The GOP right, which is to say almost the entire party, has joined with the Democratic left, which is to say a good deal of the party, in calling for a withdrawal from Afghanistan. If there were compelling reason to stay, if national security were directly involved or the chances of success were good, the president should keep ample troops in the country. But drones have riddled the Taliban/al-Qaeda infrastructure — almost 2,000 killed in Pakistan since 2006 — and can continue to press the fight. Similarly, elite and specialized troops — SEALs, etc. — can be used to keep the Taliban at bay and al-Qaeda dead.

Staying in Afghanistan will only buttress the argument of the New Isolationists. This is the larger danger. America remains the sole nation capable of playing the role of adult. The world needs us. The world will soon need us even more. China, India, Pakistan, Japan and the two Koreas are about as compatible as the Real Housewives of New York. They all either have or are capable of developing nuclear weapons. Iran is on its way. Its program could cause the Israelis to attack, and it might also prompt Saudi Arabia and maybe Egypt to go nuclear. Jordan could implode, and Iraq could come apart. Have I mentioned cyber-warfare? That’s the one that gives military planners insomnia.

Afghanistan is an odd, irrelevant place to get bogged down. We can kill terrorists but not the culture that produces them. The corruption is staggering, our lack of understanding is humbling and our war aims are incoherent. It’s time to say goodbye and save our powder for what really matters — the demons of sleepless nights to come.

cohenr@washpost.com