Because this question deals with a large subgroup within the much larger population of Europe during a major war, there is no simple single answer to this question. Each person swept up in World War II has their own tale to tell.

Nevertheless, some generalizations can be made as to why communities of Jews existing in various regions of Europe before the war started were no longer there when the war ended. These can be broken down by chronology and region.

During the period between 1933 and 1940, the official German policy toward the Jews within its sphere was to pressure them to emigrate. Hundred of thousands of Jews left Germany, Austria, and Czechoslovakia as it became increasingly difficult for them to work within their chosen professions. Because of Nationalist government policies in Poland before the start of the war, there was heavy emigration of Jews from that area as well before the war started. The destinations for these emigrating Jews included Palestine, North America, South America, Great Britain, France, and The Netherlands.

Once the war in Europe commenced with the German invasion of Poland -- followed shortly by the Russian invasion of the Eastern half of Poland -- populations in this area of the continent began to shift. Soon thereafter, German policy toward the Jews shifted as well.

When Germany invaded Poland from the west, Russia invaded Poland from the east. The Soviet also invaded and took control of the Baltic countries during this period. Nationalist movements in these countries were quickly and brutally suppressed by the occupying Communists which established the context for what happened next.

It must be kept in mind that the bulk of Europe's Jews lived in Eastern Europe before the war started and that these same areas came under Communist control when the war ended.

In 1939 France and Great Britain declared war on Germany over the invasion of Poland and in 1940 Germany invaded France. After the French surrender, the Nazis toyed with the idea of establishing a ghetto for Europe's Jews on Madagascar, a French colonial possession. This idea was abandoned because of British naval control of the Atlantic Ocean.

The following year Nazi Germany turned eastward once more and invaded Russia capturing huge areas of land.

As the German army marched eastward, and even before, the Soviets under Stalin deported millions of civilians from the areas soon to be under German control into the Russian interior--to work in large industrial complexes constructed in the Ural mountains. Up to twenty-two million persons, or one third of the local population living in these areas of occupied Soviet territory were either evacuated or conscripted into Stalin's military. Since the density of Jews in these areas were higher than elsewhere, it can be assumed that over a million Jews who would have otherwise fallen under Nazi control did not. In addition, a million Polish Jews escaped to the Soviet interior ahead of invading Germans.

Along with the retreat of the Red Army and the advance of the German Army came attacks on local Jews who did not retreat with the Communists. These pogroms were both spontaneous and encouraged by the Nazis. This happened in the Baltic countries in particular which had suffered Soviet occupation for over a year. The abuse suffered under the Soviets was closely associated in the minds of the local population with Jews who were seen as closely cooperating with the occupation. When the commissars left the Jews were attacked.

At the same time, captured Jewish political commissars and irregular guerrilla forces were summarily executed by the Nazi political police under orders from Berlin.

From the time of the invasion of Soviet territory to final reversal of Nazi fortunes in the east in 1944, National Socialism dictated that ghettoes and work camps be created in the depopulated regions in the East under German control. During this time many Jews were stripped of property and sent from German and German-occupied areas in the West to the East. Entire families were deported to eastern ghettoes and resettled there. Others were sent to work camps and factories to labor for the German war effort.

A report estimating Jewish populations commissioned by the Nazi government indicate it was believed that the number of Jews in the world at the beginning of 1943 was approximately 15.9 million and that it had dropped by 1.1 million since 1937. The number living under German control had decreased by 3.9 million through emigration and deportation. 440,000 deaths in excess of births in German occupied areas during this period contributed to the estimated decrease in world Jewry.

Starting in 1944, the Germans began conscripting labor from the ghettoes at a greater rate. German factory workers were put into military service and Jews were put into the factories to replace them. That spring several hundred thousand Hungarian Jews were taken from Hungary and sent to camps and factories around Europe to work for the German war effort. As a result many communities were scattered and family members separated from one another.

One has to keep in mind that there was a war going on. Tens of millions of people were displaced as the contending armies swept back and forth across the continent: First in 1941 then again in 1944 and 1945; This included people of many different ethnic backgrounds, not just Jews. They had no homes to which to return because of the war's destruction or military confiscation.

In the wake of the war, several political movements moved Jews in different directions. Communism in the east presented opportunities to Jews for power and revenge. Traditional Jewish charity organizations such as the Warburg's Joint Distribution Committee attempted to re-establish Jewish communities in Europe where they had existed in Western Europe before the war. Zionism actively recruited settlers and soldiers for Palestine and a future Israel among the camps of war refugees and displaced persons. Opportunities in the United States and elsewhere attracted many from Europe just as it had done before the war. Populations in Europe were not static during this time and this included Jews -- especially Jews.

Similarly for Germans; Danzig, Memel, and Koenigsberg were majority German cities when the war began. They no longer were when the war ended. What happened to the Germans who had lived there for centuries? What happened to the Jews of Europe is the same as what happened to many Germans. They became refugees with no place to call home. The trend at the end of the war and in succeeding decades was for hundreds of thousands and millions of Jews to quit Europe to go to Palestine and the United States and the far corners of the globe.

Many of those that did stay in the east became active in the various communist regimes established by the Soviets in eastern Europe. It was common that they conceal their Jewishness by changing their names in order to give these governments a more popular image. Even so, pogroms in reaction to communist rule occurred in Poland, Hungary, and elsewhere in the decades following the Communist military victories there. As a result, many Jews continued to emigrate from eastern Europe after the war ended.

In summary, what happened to the millions of Jews that once lived in Europe falls into these categories; The majority emigrated to other parts of the world; A large fraction stayed in Eastern Europe which came under Soviet control; Another fraction, probably on the order of one million died during the war in Nazi camps, in Soviet camps, in Soviet military service, in pogroms, in German anti-partisan actions which included reprisal killings, or of disease and privation.