One of the similarities one can observe between the causes of WWI and WWII is that of the role of Germany. Concerning WWI one of the greatest causes of the war can be seen as the effect of the growth of Germany had on Europe. She was a relatively new country but one that was rapidly growing financially, militarily and industrially; this threatened to disrupt the delicate balance of power achieved in Europe after centuries of conflict. However, the situation was exacerbated by her actions resulting from this new found power. The Kaiser began to feel increasingly endangered by the perceived encirclement of the entente powers and so resorted to militarism, as illustrated through the arms race, a situation which would inevitably earn the concern and anger of Britain with Germany's obsession with the growth of their navy. This along with his militaristic alliances, plans made on a war footing and aggressive foreign policy convinced Europe that a war was inevitable. Prior to the Second World War the situation was similar. For, with Hitler's rise to power there was a dramatic change in the foreign policy of Germany. Hitler believed, along with the rest of Germanys, that the Treaty of Versailles was unfair, he stressed how the nation had been tricked and did everything in his power to overturn its term; re-militarising, invading the Rhineland, forming the Anchluss with Germany. However, it was his belief in the union of all of the German race in his dreamt of "greater Germany" and the subsequent concept of "lebensraum" that was what, in reality led to war, as he began slowly to invade his neighbours and expand his eastern border, again a growing, aggressive Germany convinced the European power's that a war would be necessary to safeguard their interests.
Within this, one can observe that the main source of these actions came from the leaders of the time; Kaiser Wilhelm II and Adolph Hitler.