The Holocaust, the systematic murder of six million European Jews and millions of homosexuals, communists, Roma, and other categories of humanity by the German regime and its allies in countries and regions like Ukraine, Poland, Croatia, Belarus, Serbia, Albania, and Lithuania, enjoyed considerable active support from hundreds of thousands of collaborators. Defining "collaborator" as one who actively participates or supports the efforts of those committing treasonous or criminal acts, then categories of collaborators in the conduct of what the Nazi regime in Germany called "the Final Solution to the Jewish Problem" are numerous.
The first category of collaborator was the German, Polish, French, Ukrainian, and other nationality or ethnicity who, out of anti-semitic sentiments of his or her own, helped the German Army and secret police to locate and capture Jewish citizens of their countries with the knowledge that they would be, at best deported from their homelands and, at worse, sent to concentration camps or factories as slave labor. The number of countries from which collaborators were drawn is too long to list, but includes much of Europe, especially Central and Eastern Europe. All of these countries had large numbers of citizens who were willing participants in the Holocaust by assisting the Germans and German-allied governments to round up and deport or massacre their Jewish populations.
"Bystanders" to the Holocaust is a much broader category of people, as it refers to the millions of people throughout Europe, Asia, the Middle East, and North America who knew about the wide-scale government-sanctioned persecution of the Jews and, whether out of passive support, a sense of disbelief, an unwillingness to get involved in petitioning their governments, or a disdain for what was happening blunted by a sense of impotence, did nothing. One of the central questions regarding the Holocaust -- how much did the average German know about what has happening in his or her own country -- has largely been resolved through academic studies indicating that the vast majority both knew and agreed with or actively supported the persecution and eventual destruction of German Jews. Most of these had no direct role in the Holocaust, but either tacitly supported it or knew and simply ignored what was happening.
The United States has been accused by many Jewish-Americans of being bystanders to the Holocaust. An issue that will likely never be definitely resolved, the stance of the Roosevelt Administration with regard to the known persecution and imprisonment of European Jews, and its unwillingness to act to try and stop it, will forever haunt American history. Whether President Roosevelt could have ordered the bombing of the rail lines used to transport Jews to concentration camps, or bombed the camps themselves as a means of providing escape opportunities for prisoners, is not the question. The question remains whether such bombing missions would have, as Roosevelt's supporters contend, diverted important assets away from the fight against the German Army, or whether bombing would have done much good anyway.
Bystanders abounded across Europe. Whether our of fear of retribution for trying to save Jewish citizens, or out of passive acquiescence to German policies with regard to Jews, bystanders constituted much of the continent.