Perhaps it is the intensity of the fight that makes the difference. Everything that a nation has is put into the struggle. New powers are developed, new forces are let loose, new capacities are discovered and exploited, and these have a permanent effect. Beyond either victory or defeat they go on working; it becomes impossible for the warring nation to go back to its prewar status simply because the effort of fighting the war has destroyed that status forever.

The classic example of this is, of course, that hardy perennial of the modern book lists, the American Civil War, and Allan Nevins examines the process in an excellent new book, The War for the Union . He subtitles his book “The Improvised War,” and he is chiefly concerned here with how the improvisation took place and what it finally led to.

If ever two peoples were unprepared for war, the peoples of the North and the South were unprepared in 1861. They had to make the war up as they went along, and in the end almost nothing that happened came because anybody had really planned for it. The first year of the war is a long record of mistakes. Problems of finance and equipment had to be solved catchas-catch-can; armies had to be whistled into existence according to the obsolete military tradition of the time, which meant that in matters of discipline and training they were almost entirely out from under central control; generals had to be created out of any material that came to hand, and strategic planning (where it existed at all) was a singular blend of political considerations and dimly understood military principles, carried out by officers who in many cases tried their best to be virtually independent of the national government.

The War for the Union: The Improvised War, 1861–1862 , by Allan Nevins. Charles Scribner’s Sons. 436 pp. $7.50.

The record of the first year, accordingly, is apparently indecisive. In the East, the Union government suffered the disgraceful setback of Bull Run; in the West, it had the equally humiliating setback of Wilson’s Creek. Only in West Virginia, in Kentucky, and at isolated spots along the Atlantic seacoast did the national government record any definite advances, and these seemed to be peripheral matters that might easily have been canceled out by more extensive reverses later on.

Yet it is clear that an immense job was done. More than 500,000 men were brought under arms, a new fleet was created, the industrial mechanism to support an all-out war effort was slowly brought into being, and the amorphous enthusiasm for “restoring the Union” was somehow hardened, by slow degrees, into the grim determination that would finally insist on driving ahead to all-out victory at any cost. And amid all of this, the shape that the war would finally take was determined.

For what was taking place was in