I can't think of one war in since 1812 that was both unavoidable and existential to the country's survival. The Civil War was existential, but avoidable. The war with Mexico was not existential to the Republic, but did protect the lives of "American" settlers in Texas. Everything else after the Civil War was completely unnecessary. We should have been neutral in both World Wars or at best, played mediators in resolving issues. WWII doesn't happen without the Treaty of Versailles after WWI. The issues we have in the Middle East today is a result of the Sykes-Picot Agreement in WWI. Japan would have likely been a samurai backwater in 1900 had it not been for Commodore Perry.