Igor Markov's answer to Why does the US government spend so much on military? Jon Davis' answer to Given that China is fine without blue-water navy, why does the US need one? Being even is the Deadliest form of Warfare
Imho the most influential war in history is the Caesar Augustus' failing attempt to conquer the Germans. He sent two legions over the Limes Renano expecting to tame the german population. It didn't happened: in the Battle of Teutoburg Forest the army of 15.000 well trained roman solders was completely slayed but Germans. In a what if...
ANSWER. The most important battles of WWII were: The Battle of Britain, The Battle of the Atlantic, Pearl Harbor, The Battle of Stalingrad, The Battle of Berlin, The Battle of Iwo Jima, and the Battle of the Bulge. Every single battle of WWII was significant in some way.