How were World War 1 an World War 2 alike?

Answer 1

Note: this answer only discusses similarities. There were of course many differences not mentioned.

Both wars involved many different countries, multiple theaters of war, and extreme destruction and casualties--both with soldiers and civilians. Both were very much Total War, where the participants made production and fighting their number one priority.

Each time the UK, France, and Russia were later joined by the United States to combat Germany. Also, Italy twice began on the German side and later in the war switched to join the Allies.

World War II was in many ways a continuation of WWI because the Treaty of Versailles failed to handle Germany properly. (Basically, it made them angrier.) A primary cause of both wars was competing ideas of nationalism, which means a country--Germany in both cases, to some extent--seeks to expand or become stronger.