Could the Holocaust have been stopped?
Absolutely. My evidence for this argument is the Danes. Denmark had a relatively small Jewish population, but they collectively decided as a society that they were not going to give them up, so they hid them in their own families, changed their names, and in one evening, smuggled them out of the country to something like safety in Sweden. The Holocaust happened because the people of Germany and others did nothing to stop it. They encouraged it. They practiced an open anti-Semitism that showed the Nazis there would be no opposition to murdering the Jews.
Another way that the Holocaust could have been stopped is if Hitler himself had been stopped early on before he had gained so much control of Europe. Consider Neville Chamberlain's belief that his talk with Hitler had solved the problem, but what might have happened if Britain and other countries had taken swift action against Germany when it claimed Sudetenland?
Additionally, when reading Holocaust memoirs and histories, I have found many times when the Jewish people themselves quite possibly could have prevented the Holocaust by refusing to move to ghettos or by organizing a major resistance movement at the start of the removal of their rights. In Elie Wiesel's Night for example, he lists several opportunities that he and his family missed to escape. They were warned by other Jews and friends, and his father like other Jewish businessmen had enough money to move his family out of the area. Unfortunately, many of the Jews kept convincing themselves that each lost right would be the last and that surely it could not get any worse. I'm certainly not arguing that the Jews deserved what happened to them, but the Holocaust must serve as a constant reminder to defend our freedom and the freedom of others at all times; so that we do lose that freedom in bits until we have nothing left.
One way that it could have been stopped, or at least greatly reduced in its scope was if countries like England or the United States or France, etc., had allowed Jews to emigrate to their countries. Because anti-Jewish sentiment was rampant within these countries as well, particularly amongst the elite and powerful, very tight restrictions were placed on the numbers of Jews that were to be allowed in each year, thus preventing many Jews from escaping the persecution they faced and the beginnings of the holocaust prior to the start of the war. Even the blockade by the allies prevented food aid and other important supplies from reaching people and helped to exacerbate the situation by increasing numbers of starving, particularly those unlikely to get first priority like the Jews herded into ghettos already, etc.
As the previous poster said, there was very little willingness on the part of other countries to do anything to intercede at the beginning until their entry into the war became politically and economically expedient. No one was going to come to their aid unfortunately until it was far too late.
There are, in my opinion, two ways that the Holocaust could have been stopped.
The first is the more likely one. The German people would have had to stand up against the Nazis when the Nazis started to make the laws that discriminated against the Jews. If these early laws had drawn a lot of protest, the later extermination would likely not have happened. So the German people could have stopped it.
Once the persecution of Jews was under way, the only way to stop it was to defeat Nazi Germany. Another country would have had to invade Germany to stop Germany from persecuting the Jews. This is very unlikely to happen -- countries do not invade other countries to stop them from doing things like the Nuremburg Laws.
There are numerous reasons as to how the Holocaust could have been stopped, but here are two prominent ones.
First of all, many believe that the election of Hitler as president was a huge mistake. The election comes from many factors. First is the Great Depression which caused chaos in Germany, causing people to want a stable and strong leader. Second is Hindenberg's election of Hitler to chancellor. This gave a lot of political power to Hitler. Third of all is how the social democrats and communists did not unite to go against the Nazis together. The communists had received orders from Stalin in Russia not to form a union with the social democrats. The social democrats, on the other hand, did not consider the Nazis to be even that big of a deal, just a little too barbaric.
The second reason is on all the other countries of the world that took notice of Nazism's rise and the Holocaust. Although many organised demonstrations against Nazism, the actual country itself did little to help overthrow Germany of Hitler and the Nazis. There were also some supporters of Hitler over the world, like Henry Ford, that sponsored the Nazi's expenses.
I believe that it couldn't have been stopped earlier than it did - and prevention was impossible. It's easy to look at it retrospectively and say that other countries should have intervened before Germany had conquered so much of Europe - but it's just not realistic. Remember that America had itself to worry about considering that there was rising tensions with Japan, and leaving themselves open to attack would have been beyond dangerous. Also, Germany had the strongest military force at that time, so no single country was going to wage war on Germany alone. It was just a waiting game until there was an appropriate time to strike (when Hitler foolishly invaded the USSR).