Despite Leignitz and Kalka it was impossible. Mongols wouldn't manage to hold Western Europe because there were no steppe. Mongols could hold steppe countries (or sandy deserts) but not forests. Look at Vietnam - 100.000 Mongols entered the country, left it 20.000. Guerilla killed them. Similar thing would happen into Poland and Germany. Furthermore Mongols wouldn't easy capture Alps and Italy.
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