The book is an account of the first month of World War I, with Germany racing through Belgium and across northern France faster than the French could have imagined, yet not fast enough to accomplish the knockout blow the Germans needed if they were going to be able to move a large part of their army east to face the Russians.
By early September the German advance had been halted, but the French were too weakened to push them back out of the country, and so they settled down to the years of stalemated trench warfare.
Tuchman sets the stage by describing the military planning of the major combatants during the early years of the 20th century, including most famously the Schlieffen Plan of the German general staff.
Germany was correct in expecting the French to attack due east into the provinces of Alsace and Lorraine, that they had lost after the Franco-Prussian War of 1870. Under the Schlieffen Plan, Germany would put just enough troops at that part of the front to slow down the French advance and keep it from advancing far into Germany, but the great bulk of the army would be further north and would come pouring through (neutral) Belgium into northern France, then turning south to seize Paris from the north and west. Having taken the capital, the German forces could continue eastward (in the direction of Germany), trapping the bulk of the French army between the defensive troops on the German frontier and the massive invasion force.
The vision - as in other countries - was of a quick war: conquer France in six weeks, deal with the Russians in some short period after that, and be done. But there was an inkling that things wouldn't be that tidy.
Old Field Marshal Moltke in 1890 foretold that the next war might last seven years - or thirty - because the resources of a modern state were so great it would not know itself to be beaten after a single military defat and would not give up. His nephew and namesake who succeeded Schlieffen as Chief of Staff also had moments when he saw the truth as clearly. In a moment of heresy to Clausewitz, he said to the Kaiser in 1906, "It will be a national war which will not be settled by a decisive battle but by a long wearisome struggle with a country that will not be overcome until its whole national force is broken, and a war which will utterly exhaust our own people, even if we are victorious." It went against human nature, however - and the nature of General Staffs - to follow through the logic of his own prophecy. Amorphous and without limits, the concept of a long war could not be scientifically planned for as could the orthodox, predictable, and simple solution of decisive battle and a short war. The younger Moltke was already Chief of Staff when he made his prophecy, but neither he nor his Staff, nor the Staff of any other country, ever made any effort to plan for a long war. Besides the two Moltkes, one dead and the other infirm of purpose, some military strategists in other countries glimpsed the possibility of prolonged war, but all preferred to believe, along with the bankers and industrialists, that because of the dislocation of economic life a general European war could not last longer than three or four months. One constant among the elements of 1914 - as of any era - was the disposition of everyone on all sides not to prepare for the harder alternative, not to act upon what they suspected to be true. (pp. 26-27)A long war may have been the logical conclusion from looking at how ready everyone was to fight, but it was also unimaginable. The Napoleonic wars had dragged on for nearly 20 years, but they were a series of relatively short campaigns with considerable periods of peace in between. There was nothing in history to prepare people for the reality of what World War I turned out to be, even if a sober analysis would have told them it was coming.
Disruption from climate change is the logical conclusion from looking at the evidence - arguably more logical than someone in 1906 foreseeing a long war - and yet the consequences are similarly unimaginable. Well, not unimaginable, but somehow unreal. We've never lived in a world as warm as the one we're making for ourselves. That disrupted future is only an idea. That puts it fundamentally at a disadvantage against the mostly-normal reality in which we still live.
The disruption scenario is pretty horrible and requires an act of imagination to remain present in our minds, so it is psychologically easier "not to prepare for the harder alternative, not to act upon what [we] suspect to be true."