Can’t maintain your best behavior?

Blame entropy

Woodrow Wilson believed in the responsibility of great powers like the United States to intervene in the affairs of other countries where necessary to preserve order. But he knew such intervention would be an endless task. He once likened it to painting a fence. You can't paint the fence once for all; the paint chips and wears away. You have to go back and repaint, again and again. 

Peace settlements at the ends of wars have the same property. After fighting themselves to the point of resolution or mere exhaustion, the belligerents sit down and agree to stop. Even in those rare cases where the peace terms are completely satisfactory to both sides, the satisfaction does not persist. New demands emerge, straining the terms of the settlement. New leaders, perhaps, take over and, feeling no investment in the settlement, decide to break it. A new war results. 

Within countries, similar patterns mark politics and private life. Opposing political parties agree to a budget for this year, but next year they have to renegotiate the whole thing. Labor unions and company management return to the bargaining table on a regular basis, thrashing out the same old issues and some new ones. People fall in love and get married, vowing to remain true until death, only to revise their estimates of their partners and decide, in many cases, to void the vows long before decease. 

In simple terms, no human solution to any human problem lasts forever, and many solutions don't last for very long. 

Why is this so?  

One could cite the fickleness of human nature, the fallibility of human judgment, the turnover of human generations. Humans are simply hard to please and impossible to please for long. 

Another way of looking at it is by reference to the second law of thermodynamics, which states that disorder—entropy—tends to increase. Hot objects cool down; cold objects warm up. In a hot object, the molecules have a higher average energy than the molecules of the surrounding material; that's what makes the object hot. The orderly arrangement of energetic molecules in the object and sluggish molecules in the surrounding material gives way to the disorder of molecules of various energy levels jumbling together as the object cools off. 

One way of interpreting this result is to say that the energetic molecules transfer some of their energy to the sluggish ones. Such does indeed happen. Another way is to view the situation statistically. In a given substance—a gas, say—there are energetic molecules and sluggish molecules. A hot gas has a greater proportion of energetic molecules than a cooler gas. If the hot gas and the cooler gas can be separated by a partition, they will tend to retain their different temperatures. Remove the partition, though, and the difference in proportion goes away. And the chance of the molecules resegregating on their own is vanishingly small. The gas will remain at the tepid new temperature. 

Human beings are not molecules of gas. They are distinct individuals and have minds of their own. Yet taken in groups, they display similarly statistical behavior. For a village to be peaceful on a given night, everyone in the village has to mind his or her manners. Statistically, there's only one way for this to happen: everybody acts properly. But there are many ways for disorder to break out; any man or woman can break a window or steal a car. So even if a police crackdown or a fit of civic virtue causes the village to be peaceful one night, the chances of the calm persisting decrease with each passing night. 

Absent, that is, an application of force or persuasion. The second law of thermodynamics allows that the tendency toward disorder in a system can be offset by the addition of energy. A boiling tea kettle will continue boiling as long as the stove is turned on. The point is that energy has to be added to the system to prevent the disorder. 

Which brings us back to Woodrow Wilson. The fence doesn't stay painted by itself; the painters have to return and repaint it. The peace treaty doesn't enforce itself; guarantor powers have to enforce it. If they become weary or distracted, human entropy takes over, and disorder resumes. 

Historians and many others lament the inability or unwillingness of humans to learn from the mistakes of our predecessors. Wars recur because we forget their cost.  

That is certainly part of the problem. But it's not just human nature that works against peace and order; it's nature itself. The ways things can go wrong greatly outnumber the ways they go right. We can beat the odds for a while, but entropy always wins in the end.