We found the concentration camps in April 5, 1945. However, the war began in 1939. With the amount of concentration camps (both major and minor), how is it possible to have not came across one of these camps for so many years?
I'm matching up the battle locations via this map, and it would seem that it wasn't until 1943-1944 when they would have come across these camps. However, by the time they found the first camp, it was 5 months away from the end of the war.
So, how did they hide these camps for so long, and how is it possible to not have stumbled across even a "minor" camp?
Ok, to answer this, it is important to remember that the Allies knew about the Concentration Camps before the first ones were liberated.
The existence of the camps was not something Germany was hiding and it all had been well publicized as early as 1933 that such camps existed. Additionally, the Western Allies received intelligence mainly by the Polish resistance during the war that described the camps and the atrocities committed there. People like Witold Pilecki, a Polish officer who entered and escaped from Auschwitz and subsequently wrote a report about it, lead to the Western Allies being informed about the camps and what was happening there. The Soviets as well had their sources and since so much of the killing took place on their soil, were also well informed about what was going on.
The other thing is that with all the camps you see on that map some didn't exist anymore when the Allies drew near. The Operation Reinhard Death Camps e.g. were closed down in 1943, torn down, the buried corpses burned and farmsteads build where the camps once had stood.
The first major camp liberated was the Majdanek camp in July 1944. When the Soviets reported what they had found, other Allied governments were careful in their belief because they suspected that some of it might be Soviet propaganda. Similarly, when Soviet forces liberated Auschwitz in January 1945. They choose not to publicizes a good junk of information which was a major factor contributing to the surprise of American troops liberating camps in April 1945.
Additionally, what the Americans found is not exactly representative of the realities of the camps in the earlier years. The Nazis evacuted the camps before oncoming allied troops and therefore many of the camps within the Reich were completely overflowing with inmates who after weeks of walking from one camp to another were in very bad condition.
I hope this answers your question and please don't hesitate to any further questions you might have.
Sources:
Stefan Hördler: Ordnung und Inferno. Das KZ-System im letzten Kriegsjahr. Göttingen: Wallstein, 2015.
Evan's Third Reich Triology.
Karin Orth (1999). Das System der nationalsozialistischen Konzentrationslager. Eine politische Organisationsgeschichte. Hamburg Edition, Hamburg.
Nicholaus Wachsmann: KL: A History of the Nazi Concentration Camps, 2015.
