Were there historic tensions among the Czechs, Slovaks and Magyar peoples? How were the expellees accepted and integrated into Hungarian society?
Which postwar are we talking about (WWI or WWII)?
Hungary In World War II: Caught in the Cauldron by Deborah Cornelius does a good job of covering Hungarian politics from the end of World War I to the Communist takeover in 1946.
Simply put, on several different occasions during the interwar era, Hungary tried to push north and reclaim the heavily Hungarian southern half of Slovakia. Opposition to Hungarian irredentism was critical in forming the "Little Entente" alliance between Czechoslovakia, Romania, and Yugoslavia during the interwar period. During the war, Hitler awarded his Hungarian allies with the southern half of Slovakia, turning the rump state of the rest of Slovakia into a puppet state run by a Nazi-friendly priest.
After the war, the (democratic, liberal) government of Czechoslovakia wanted to make sure it would never be threatened again by irredentism and tried its hardest to expel Germans and, to a lesser extent, Hungarians from their borders. This work was almost completely finished by Czechoslovakia's 1948 coup that installed a pro-Soviet Communist government.
This map can help out. Remember Hungary was a Nazi ally and almost all of that land was awarded to Hungary by the Nazis as a present for its loyalty. Except for Carpathian Ruthenia (in the northeast) all of that land was either majority or near-majority Hungarian. Needless to say, all of Hungary's neighbors after the war had no desire to let Hungary continue to have claims on them.
