During the months preceding the attack on Pearl Harbor, the war in Europe had essentially boiled down to a contest between the Axis Powers of Germany and Italy, and against them, the Soviet Union and Great Britain.

Six months before Pearl Harbor, Germany had launched an invasion of the Soviet Union, its erstwhile ally. By December 5, two days before the Japanese attack on Pearl Harbor, German armies had advanced to within 5 miles of Moscow.

Hitler had decided to postpone a cross-channel invasion of Britain itself until his armies were able to defeat the Soviet Union, but Germany was still fighting Britain through aerial and missile bombing, and was engaged against Britain on the seas, as well as elsewhere in the British Empire, as in North Africa. In South Asia, Britain was also defending its colonies and commonwealth against Japan. On the face of it, especially in the long term and even with Lend-Lease aid from the United States, it is difficult to see how Britain could have continued the war without the entry of the United States into the conflict on its side. Presumably, Winston Churchill would have had to sue for peace, or endure a German invasion of the British Isles once the Nazis had consolidated their military strength in Europe.

That did not happen, of course. After Churchill heard that America had been attacked at Pearl Harbor, he rushed to a secure telephone to call Franklin Roosevelt. His Memoirs of the Second World War relate the following:

In two or three minutes Mr. Roosevelt came through. "Mr. President, what's this about Japan? "It's quite true," he replied. "They have attacked us at Pearl Harbor. We are all in the same boat now."

No American will think it wrong of me if I proclaim that to have the United States at our side was to me the greatest joy. I could not foretell the course of events. I do not pretend to have measured accurately the martial might of Japan, but now at this very moment I knew the United States was in the war, up to the neck and in to the death. So we had won after all!

Yes, after Dunkirk; after the fall of France; after the horrible episode of Oran; after the threat of invasion, when, apart from the Air and the Navy, we were an almost unarmed people; after the deadly struggle of the U-boat war—the first Battle of the Atlantic, gained by a hand's-breath; after seventeen months of lonely fighting and nineteen months of my responsibility in dire stress. We had won the war. England would live; Britain would live; the Commonwealth of Nations and the Empire would live.

How long the war would last or in what fashion it would end no man could tell, nor did I at this moment care. Once again in our long Island history we should emerge, however mauled or mutilated, safe and victorious. We should not be wiped out. We should not be wiped out. Our history would not come to an end. We might not even have to die as individuals. Hitler's fate was sealed. Mussolini's fate was sealed. As for the Japanese, they would be ground to powder.

United States forces played a direct role in defeating Germany, but also forced Hitler to keep huge military forces in Western Europe rather than sending them to reinforce his armies fighting against the Soviet Union, where they would likely have been a decisive factor against the Soviets. Instead, the German invasion of Russia failed after the effort that culminated at Stalingrad, and the German forces in Western Europe were eventually pushed back anyway, beginning with the landings at Normandy.

Your questions are hypothetical. They invite speculation. One of the delights of the alternate history genre of fiction is that its authors generally expend some considerable effort on the notion of history itself, especially the way in which history unwinds out of a skein of causes both large and small. In these novels, large and familiar causes and conditions and forces roll out across the world, but small human details, such as a missed appointment at the Reichs Ministry, an overlooked telegram, Hitler's mistress Eva Braun's choice of perfume on a fateful day, or a random batch of sunspots that interferes with a particular radio transmission, sometimes cascade into a vastly different history than the one with which we are familiar.

Very few Civil War farmers went home to take care of the crops and then returned to fight. The American Civil War was far longer and more destructive than virtually anyone expected. At the outset of the war, many eager volunteers signed 90-day papers, evidence of the widespread belief that the war would be brief and nearly bloodless. Instead, the struggle stretched nearly four full years. Between 1861 and 1865, the Union and Confederacy mobilized more than three million men for the fighting and suffered some 600,000 deaths. The war remains the bloodiest conflict America has ever fought.

One of the reasons for the dramatic increase in the length of the war, the size of the armies, and the magnitude of the casualty rolls was the technological improvements in agricultural production that occurred in the decades leading up to the war. In the eighteenth century, the food demands of the population at large served as an important limit on the amount of manpower that could be mobilized into the army and on the length of time those armies could be maintained in the field. In those years, a nation that diverted a large proportion of its agricultural labor force to the military and kept it in the field for months at a time risked deprivation and starvation on the home front.

A bevy of technological improvements that appeared in the first half of the eighteenth century—iron and steel plows, seed drills, cultivators and, most importantly, mechanical reapers and threshers—made it possible to increase the amount of food that could be produced manyfold. By the mid-1800s, two men equipped with a horse and the new agricultural machinery could produce as much grain in a day as twenty men could harvest by hand using sickles in the late 1700s. Together, these changes dramatically altered the proportion of the population required for agricultural production. Three-quarters of American workers labored on farms in 1820; by 1860, fewer than three in five worked in agriculture.

The massive shift in the demographics of food production helps explain how the Civil War armies could place so many men under arms and keep them in the field for years on end. The massive armies that contested the huge battles between 1861 and 1865 could fight so long in part because their members were not needed at home to plant and harvest crops: the divisions could stay on campaign without threatening the larger society with starvation. And the substitution of mechanical advantage and animal effort for human muscle meant that farm labor no longer required adult men. Younger children and women could now perform many of the tasks that formerly demanded male workers. In the south, the labor of four million African-American slaves helped support the armies in the field and freed the white population to continue fighting. While neither side offered exemptions to farmers, the South did institute the Twenty-Negro Law, which released a plantation owner or overseer whom managed more than twenty slaves from military service—powerful evidence not just of slavery’s importance in maintaining the Southern economy and war effort but of the persistent fears of slave insurrection.

This is not to say that soldiers never returned to their homes and farms during the war. Furloughs for the troops were not uncommon, often doled out as a reward for service. (In the winter of 1863-1864, a Union Army desperate for reenlistments offered a 30-day furlough to any soldier willing to sign on for an additional three-year term.) But those furloughs rarely coincided with the agricultural cycle: campaigning season usually ran from the spring to late fall—precisely the months of planting and harvesting—and armies found it easiest to grant furloughs once the armies had gone into winter quarters. Soldiers generally viewed furloughs as a chance to visit loved ones and to escape the stifling discipline of army life for a brief moment.

In the South, the deprivations of war affected individual soldiers more acutely. Non-slaveholding white farmers—those who farmed small parcels of land for sustenance, as opposed to large planters growing cash crops—frequently joined the army in the belief that they were defending homes and families from Yankee invasion. As the war dragged on, and Federal troops pushed deeper into the Confederacy seizing crops and blocking transportation lines, many of those families suffered intensely from the scarcity of basic supplies. When letters arrived from home telling of wives, children, and siblings threatened with starvation, many of those rebel soldiers elected to leave the ranks and return home to provide for their families. Those desertions—officially a capital crime in the eyes of the military system—became more and more widespread in the last years of the war; historians estimate that as many as one in three Confederate soldiers had deserted by the final months of the war. Very few of those deserters returned voluntarily to continue the fight.