WWI and WWII can be compared in terms of causes, sides involved, main fronts or theatres of war, military operations, weapons/warfare, basis for peace, and global effects.
Nationalism may be said to be n ∘1 cause of both WWI and WWII. However, whereas WWI was mainly brought about because of Germany's urge to prove she was better than UK, WWII was brought about by a different kind of nationalism, i.e. a nationalism of re-assertion: Germany wanted both to take revenge and to show the world that she could "emerge from the ashes."".
As in WWI, imperialism also played a great part in bringing about WWII: one of the main causes of the Second World War was Mussolini's and Hitler's hunger for territories. Moreover, and just like in WWI, several alliances were made before WWII, which also contributed to the starting of hostilities: in 1934, Germany and Poland signed a non-aggression pact; later on, Hitler and Mussolini signed the Anti-Comintern Pact together with the Japanese, the main objective of which was to limit communist influence around the world (the alliance became known as the Axis Powers). Finally, in August 1939, Hitler and Stalin signed the Nazi-Soviet Pact, by which the two arch-enemies agreed not to attack each other and to divide Poland between them (though this second clause was kept secret).
Two blocks of countries fought against each other both in WWI and in WWII: in WWI, the Central Powers (Germany, Austria-Hungary, Turkey) confronted the Allies (France, UK, Russia),1 and in WWII, the Axis Powers (Germany, Russia2, Italy, and Japan) fought against the Allies (UK, France, and USA).
3. Main Fronts or Theatres of War.
Both WWI and WWII were fought on many fronts: on land, sea and in the air, and both wars affected everybody, including civilians at home, i.e. both wars were "total-. Besides, and as a result of the fact that so many men were away at war, women had to take their places in arms factories and on buses and trains.