France's military history is littered with cautionary tales and glorious triumphs. And the secret differentiator between the two may be age.

The average age of Napoleon's generals was 41, and many of the brightest were even younger. Jean Lannes was named a general at 27, and a field marshal at 35. Andre Masséna was named a general at 35. Louis-Nicolas Davoust was named a general at 23 (really), and a field marshal at 34. Joachim Murat, Napoleon's legendary cavalry commander, was named a general at 29.

By contrast, in 1939, when France started what would be the most serious debacle in its history, the supreme commander of its armed forces was Maxime Gamelin, age 67. Before the end of the Battle of France, he was replaced by Maxime Weygand, 73. France's only World War II victories were won by a young general, who had previously written a prophetic book on blitzkrieg tactics, by the name of Charles de Gaulle.

This is a pattern so often repeated in military history that you can't help but ask, "When will they ever learn?" A military force wins a series of victories. After doing so, it becomes cocky, set in its ways, sure that its tactics will work forever. A hungrier force comes up with new and unexpected tactics. The older force cannot adapt. It is defeated. The phenomenon is so well known that "generals fighting the last war" has become a common expression.

We should always be wary of over-generalizations. Obviously, some of the world's most daring and innovative people are senior citizens, and some of its dullest and most conservative are in their 20s. But broadly speaking, these are exceptions that prove the rule. It seems hard to deny that there is indeed an inverse correlation between age and willingness to try new ideas, and between age and aggression, which are among the most critical features for military commanders.

And that brings us to America. Today, the U.S. military seems both as strong as ever and as weak as ever. As strong as ever because it has no great power rivals that can even hope to match its conventional strength; and because it has the most aircraft carriers, the best technology, and unattainable command of the skies. And yet, no one but a fool would claim that the U.S. military's recent war-making record is sterling. As Thomas E. Ricks has argued, a great many of the U.S.'s military failures in Iraq and Afghanistan are directly attributable to poor generalship.

The U.S. military is the most powerful fighting force in history. And warfare is changing faster than ever. The entire world depends on the U.S. military for security. Whatever the "next war" ends up being, we will all be sorry if U.S. generals are fighting the last war while it happens.

I don't know what the "next war" will be, so I can't make tactical recommendations. Instead, I can make a recommendation that will bias the U.S. military toward more inventiveness, more risk taking, more daring. Is it sure to work? Of course not. Will it make a true difference? Is it necessary? I believe so.

That recommendation is simple: The U.S. military should have a firm retirement age of 50 for officers.

This would be a sea change. It would mean the chairman of the joint chiefs would be in his mid-40s, instead of 62, as he is today. The career path would be compressed to an astonishing extent. It is not just generals who will be (much) younger; it will also be every type of superior officer.

There are, of course, laws in America that ban age discrimination. There are very good reasons for these statutes, and I don't propose changing them anywhere — except in the military's officer ranks.

Now, you might be concerned that my plan will promote people past their level of competence. Don't worry about that. Bureaucracies promote people to their level of incompetence; startups and adventurous militaries promote people past their level into competence, into jobs that they must grow into as they do them. A very common feature of military campaigns, especially successful ones, is officers being promoted very early due to high rank turnover, whether due to dismissals or death in action.

Plus, the U.S. military needs more accountability for senior officers. Some senior officers will not be ready for significant command by their mid-30s. Replace them!

Aren't there a lot of very valuable old commanders? Of course. As I said earlier, there are exceptions to every rule, and there are or should be a lot of exceptions to the current, never-justified, unwritten rule that says senior commanders should be in their 50s or 60s. The point of setting a general rule of this type is not to catch every single eventuality in its net. It is to find a way to shape incentives and culture and probabilities to improve the organization.

One effect of this new rule is that, given the shorter career span, and, therefore, a much more brutal "up or out" promotion system, younger ambitious officers who want to make general very fast will be more incentivized to try to stand out, to try new things and methods.

I realize, of course, how audacious my proposal is. Almost as audacious as Napoleon.