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...
Outcome: The result of the battle was inconclusive, but the north did win a strategic advantage. 23,100 casualties. Significance of the Battle of Antietam: The Battle of Antietam forced the Confederate Army to retreat back across the Potomac River. President Abraham Lincoln saw the significance of this and issued the famous Emancipation Proclamation on September 22, 1862.