During and after the American civil war, vast agricultural lands were opened to settlement, largely due to improved steel plows that allowed farmers to plow lands covered by tough prairie grasses. America then become a huge exporter of cheap grains. The same thing happened in Russia. Rapidly industrializing Germany became a major customer for these grains. Everyone would seem to have benefited from this process except for the German Junker class, whose wealth was based on their vast agricultural estates in eastern Germany. They could not compete economically with these cheap imports. Since they could not compete with cheap grain imports, and since they controlled the German government, they arranged for agricultural subsidies. They were not happy with the results though, especially when they became aware of the vast fortunes being made by their close cousins (literally) in England from the oil industry in the Persian Gulf. They wanted to get a share of this new wealth but were shut out by the English, Americans, French, Dutch and Italians who had gotten there first ( you will notice that these nations formed most of the Entente who opposed Germany in WW1).
Under the control of the Junkers, the German government then developed a close relationship with the Ottoman Turkish government. The persian gulf lands that were the object of everyone's attention were technically a part of the Ottoman Empire. The Ottoman Empire (then referred to as the Sick Man of Europe) was ignored, as the powerful oil companies moved in. The English got there first, having secured a concession to drill for oil in lands bordering the Persian Gulf. The Americans were next, having secured a concession to drill for oil in the interior. To the horror of the English, most of the oil proved to be in the interior. According to the Aramco Handbook, the Seven Sisters (as the major oil companies of the time were referred to) threatened to drag their host countries into war. There was no oil company present that was rooted in Germany. War was averted, according to the Aramco Handbook, by an Armenian with Turkish citizenship who offered to broker a settlement that would be to everyone's satisfaction in return for a 5% stake. This was accepted by the Seven Sisters and he became known as Mr. 5%. This took place around 1905. This threatened war was not against Germany but rather among the "Democracies", even though we are assured that democracies are always just and un-warlike.
Having been excluded from a share of the Persian Gulf pie, the Junkers took the tack of trying to build up the Ottoman Empire. At considerable cost to German taxpayers ( who were not planned to be beneficiaries), the German government announced plans to build the Berlin to Baghdad railway. This caused considerable concern to the "Democracies" but this concern became frantic when the German government subsequently announced it's intention to extend the railway all the way to the Persian Gulf. Suddenly Kuwait declared it's independence from the Ottoman Empire, blocking access to the Gulf and a British fleet steamed into the Gulf, threatening war against anyone who did not recognize Kuwait's independence. This fleet was paid for by British taxpayers, who were likewise not intended to be the beneficiaries of the money to be made from Middle Eastern oil.
Let me make an aside here. It is my contention that wars are not initiated by humane grey haired sages who hold that war is a terrible thing but that it will confer the greatest good to the greatest many. Wars are initiated by powerful cabals who have seized control of their countries and utilize the lives and taxes of fellow citizens to further their ends (money and power). The ordinary citizen is not expected to benefit from these enterprises. They are just expected to pay for them and to die for them. The cabal that has controlled the United states for over a century consists of the families that control the oil industry. As rationales for going to war, the citizens are fed simple slogans, soundbites as it were, that have nothing to do with the actual reasons for going to war. Incidents are manufactured like the Gulf of Tonkin Incident, the Zimmerman Telegraph, Weapons of Mass Destruction, the sinking of the battleship Maine. In his book. "Strategy", Liddell Hart describes how Roosevelt manipulated Japan into attacking Pearl Harbor. To Roosevelt, Japan was a minor irritant. The real reason was to provide a pretext for entering the war against Germany. Once war was declared, 9/10 of American resources were devoted to making war against Germany. We have seen a similar tactic recently. We were attacked by Afganistan but 9/10 of our millitary resources were spent on a war against Iraq. Sometimes a tempting target is offered. The rationale for the first Iraq war was the invasion of Kuwait by Iraq. However, Saddam Hussein made no move against Kuwait until he was reassured by ambassador Glasspie that Kuwait did not represent a strategic interest to the United States.
Otto von Bismarck once remarked that nation states do not possess morals and ethics, just interests. I think that he was right. None of the leaders of the participants in WW1 and WW2 engaged in war as a moral crusade. Yes, the Nazis would make a dog want to vomit. But the Soviets were no better. All of the participants engaged in mass murder of civilian populations, both during and after the war.
WW1 started with the assassination of Archduke Franz Ferdinand and his wife, Sofie Chotek by a Serb semi-governmental institution. If national governments possessed morals and ethics then they would all have ganged up on Serbia and kicked the crap out of them. Instead, when Austria-Hungary attacked Serbia, Russia, Britain and France declared war against Austria-Hungary. Germany, allied to Austria-Hungary, then declared war against the Entente. A question has arisen in my mind. How did a little banana republic like Serbia think that it could get away with this assassination. It is my suspicion that they were put up to do it by the Entente, that they were promised Bosnia, Croatia and Slovenia with a successful conclusion of the war. And I don't think that the dismemberment of Austria-Hungary was the ultimate goal. The ultimate goal was to draw Germany into war. German power, with the expansion of it's industry, had been growing by leaps and bounds, as had it's involvement with the Ottoman Empire, training Ottoman soldiers, supplying weapons, etc. It was time to cut Germany down to size.
With the successful conclusion of WW1, Austria-Hungary was dismembered and it's territory was parcelled out to pathologically nationalist successor states. Millions of Germans and Hungarians suddenly found themselves as unwanted and despised minorities in these new states. The Suedetenland (the German portion of Bohemia and Moravia) was one of the most industrialized regions of the world. Formerly they produced consumer goods for a market of 53 million people (the population of the Austro-Hungarian empire. Now their market was restricted to the 13 million people in the new Czechslovak state. The Suedeten Germans pleaded with Eduard Benes to negotiate trade treaties with Austria, Hungary and the other successor states so that their goods would not be excluded from these markets. Benes was not interested. Germans were not permitted to bid on state contracts. 1,700,000 Hektars of land in the German and Hungarian areas were seized for redistribution with very little compensation. Most of these lands were turned over to Czech colonists who were established in colonies in what had been 100% German and Hungarian territories. In response to criticism, Benes announced plans to divide the state into cantons, like Switzerland, but nothing ever came of this. Germans were very much treated as second class citizens. In desperation, the actions of Benes drove Konrad Henlein, the leader of the Suedeten Germans, into the arms of Hitler.
Ethnic cleansing was very much in the mind of the Polish leadership. Of 3 million Germans who found themselves on the wrong side of the border when the Polish Corridor was created, one million were thrown out of their homes in the period between the wars. According to Carl Tighe in his book, Danzig, the Polish secret service provided financial support to the Irgun. It is not that they liked Jews. They hated them. It was their hope that the Irgun would throw the British out of Palestine. If that happened, the Polish leadership planned to throw all 3.5 million Jews out of the country.
The rationale advanced for the ethnic cleansing of Eastern Europe's German population was that Germans minorities formed an indigestible population and were a source of problems for the host states. There may be some truth in that but you can hardly expect a different outcome when you treat a minority population so poorly.
Next, I will describe in some detail how the Ethnic cleansing was achieved. A good source of information is Alfred Maurice de Zayas' A Terrible Revenge