"Magna Germania" – to them this is "ancient German land" OR "Deutschlands Europa Sendung" – only we Germans can make Europe great and thus it is our duty to do so. But basically, Germanic chauvinism.
They might try a modern version of the Kulturkampf.
The situation leading to both WW1 and WW2 was a power keg of sort and thus war was, in a way, inevitable. But notice that it is the Germans that fired the first shot each time. Many German soldiers who fought in WW1 (like Hitler) were ready and willing to go to war again in the 1930s though they fully well knew the massive losses that the war brought and not just to Germany but all of Europe. Today's German nationalist, like the NPD, lack even this perspective. To them, war is Call of Duty on XboX.
Good thing that the current German government is not pushing for such irredentist measures, though they do not care much for the Slavs under their administration. Poland, on the other hand, has made a special exception for the German minority and granted a seat to their political party in the Sejm as a means to get some representation. Germans are 0.4% of Poland's population and a political party needs at least 5% nationwide support (in at least one national election) to be considered "official" so this is a pretty good exception that was made for them, since they are a distinct group.
Lastly, those who don't learn from history are doomed to repeat it. Delusions of "Magna Germania" are this, pure and simple.