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...
One of the most important historical events is the legendary First Siege of Sevastopol, when the Russian Army defended the Fortress of Sevastopol during the Crimean War. The allied army of Ottoman Turkey, Great Britain, France, and Sardinia brought 60 thousand troops to take over the city of Sevastopol.