Bangladesh, Sept. 7 -- War on a map is a calculation. War on the ground is a test of endurance.

Leaders may enter a conflict believing they understand the balance of power, the enemys vulnerabilities and the likely path to victory. But once the shooting starts, assumptions collide with reality. A ruler may know when he is going to war; he rarely knows when-or on what terms-he will come home.

That is the first lesson of the prolonged conflicts now confronting Donald Trump and Vladimir Putin.

Wars often begin with the ambition to break an adversary. If that decisive victory does not come, the objective gradually changes. The war becomes an exercise in exhaustion: not defeating the enemy outright, but persuading him that continuing to res...