This is a complicated and highly controversial question, both in Holocaust studies and contemporary Poland. Occupied Poland was in a unique position during the Holocaust, as the epicenter of the Final Solution and the place where the majority of Holocaust victims were killed. About 90% of the 3.5 million Jews living in Poland in 1939 were killed during the German occupation (the majority of the survivors fled or were deported into the Soviet Union prior to the German invasion; the death rate for Poles living in German-occupied Poland was about 98%). However, assertion that occupied Poland was the only place where this could have taken place is an inherently unfalsifiable claim, because the counterfactual can't be confirmed empirically. Instead of trying to answer a question that isn't really valid as posed, I'm going to try to explain why the situation in Poland was unique and why it became the central focus of the Holocaust.

There are a couple of factors that historians have identified as important determinants of the fate of Jews in occupied/Nazi-aligned Europe. The first is how intact the prewar political structures of that country were; countries that were more independent from German influence and retained more of their prewar political structures tended to have higher survival rates than those that were partially or fully incorporated into the Nazi state. The second is the degree of collaboration among the local population, which is fairly self-evident; countries where the local population collaborated more extensively had higher death rates.

Poland, of course, was more politically fragmented than any other country that was occupied by Nazi Germany. The Nazis were fundamentally hostile to the existence of a Polish state and viewed ethnic Poles, like all Slavic peoples, as subhuman (Untermenschen). After Germany defeated Poland in the September 1939 war, some parts of Poland (mainly northern and western Poland) were directly annexed into the Nazi state; much of this territory had been part of the German Empire prior to World War I. The remainder of the Polish territory that was in the Nazi occupation zone was converted into a semi-colonial territory called General Government. The prewar Polish government was forced into exile in London, while the Nazis completely dismantled the Polish state, replacing Polish political leaders with Germans right down to the local level. Many Polish intellectuals and political leaders who weren't able to flee the country were murdered by the Germans in the first months of the occupation (the so-called "intellectual action" or Intelligenzaktion). As a result, the protective effect of prewar state structures for Polish Jews was completely nonexistent.

The Nazis began killing Polish Jews, along with other non-Jewish Poles (including intellectuals and people with disabilities), almost immediately after the invasion. This was the first phase of the Holocaust in Poland, sometimes known as the "Holocaust by bullets". The second phase began almost simultaneously, with the creation of ghettos in most cities and large towns. Jews were forced to live in these ghettos permanently, only being allowed out to work, and marked with symbols (e.g. the Star of David); these processes of isolation and identification are now recognized as a key step in the process of genocide. Most of the Jews living in cities in the German occupation zone had been forced into ghettos before the invasion of the Soviet Union in June 1941; Jews living in the former Soviet occupation zone who weren't killed immediately were also forced into ghettos in 1941 and 1942. Death rates in the ghettos were high, primarily due to starvation and disease, which were exacerbated by the overcrowded and unsanitary conditions in the ghettos. During this time, the Germans also established concentration and labor camps throughout occupied Poland. These included many of the better-known concentration camps in central and western Poland, such as Stutthof, Chelmno, and Auschwitz I. Both Jews and non-Jewish Poles were sent to these camps to perform forced labor or as punishment for real or imagined criminal offenses.

The major turning point in the Holocaust in Poland (and the Holocaust in general) was the invasion of the Soviet Union. At that point, the primary Nazi policy toward Jews changed from discrimination, ghettoization, and forced labor, followed by the vague "deportation to the East" at an indeterminate point, to a process of systematic extermination. Over a million Jews were killed (primarily by shooting) in the occupied Soviet Union (including the former Soviet occupation zone in Poland) during the German invasion, which was a marked radicalization in Nazi Jewish policy. At the same time, German officials in Berlin were discussing plans for the "Final Solution to the Jewish Question". On 31 July 1941, Hermann Göring instructed Reinhard Heydrich, on Hitler's orders, to develop a protocol for the Final Solution, which was unveiled at the Wannsee Conference on 20 January 1942.

The "solution" Heydrich presented was the deportation of Europe's Jews to purpose-built extermination camps, where they would be murdered on an industrial scale. The six largest extermination camps (Auschwitz II-Birkenau, Belzec, Chelmno, Majdanek, Sobibor, and Treblinka) were all located on the territory of occupied Poland. The two main methods of killing used in those camps, carbon monoxide gas from engine exhaust and hydrogen cyanide gas produced with Zyklon B, had been tested at Chelmno and Auschwitz, respectively, in the fall of 1941. Auschwitz and Majdanek used Zyklon B in their gas chambers, while the other camps used engine exhaust (from a mobile gas van at Chelmno and stationary gas chambers at the other three camps).

Auschwitz, Chelmno, and Majdanek had already been in operation as concentration and labor camps before the planning for the Final Solution began, while Belzec, Sobibor, and Treblinka were constructed solely as extermination facilities. The latter three camps were used during Operation Reinhard, the extermination of the Jews in the General Government, which was the most concentrated phase of killing during the Holocaust; more than 2 million of the 3 million Polish Jews murdered during the Holocaust were killed in the space of less than two years during Operation Reinhard (Jews of other nationalities were also murdered at these camps, but their primary function was the annihilation of Polish Jews). By the fall of 1943, this operation was largely completed and the three Operation Reinhard camps were subsequently dismantled to disguise the crimes that had taken place there. The three other camps remained in operation until they were liberated by the Red Army. Auschwitz in particular became the central killing site for Jews from other parts of Nazi-occupied Europe, including Western Europe, the Balkans, and Hungary. As a result, the majority of Jews (Polish and non-Polish) who were killed during the Holocaust were killed on the territory of occupied Poland.

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