Japan declared war on the US by pre-emptively bombing pearle harbor in an attempt to permanently cripple the US Pacific fleet. The president at the time knew it was coming via intel and forward radar installations and let it happen to maximize the political mobility of the act.

After WW2, the US had been drug into 3 seperate foreign wars at that point; the Spanish war, WWI and WWII. Spanish war we were attacked at the southern tip of Texas and won that one before an invading army could get into the states, Both WWI and WWII The US entered because their merchant fleet was being attacked. In WWI a luxury cruseliner was sunk and that served as the tipping point, and in WWII our mechant fleet escort ships were attacked and the tipping point was a Torpedo fired at the Greer, a military escort ship. Basically, once you begin attacking the merchant fleet, your next step is to attack ports and port cities to stop material from getting on boats and to stop boat production. The Japanese harbored expansionist\empirial ambitions and knew they'd come into conflict with the US, hence the reason they had built a huge pacific fleet. That doesn't just happen overnight, they had a lot of build up of ships and material. Once the US entered the European theatre officially, the Japanese Military command became worried the US might attack them due to their alliance with Germany\Italy, hence the pre-emptive strike. Had Japan not attacked pearle harbor, had stayed in Asia, and had not built an alliance, a good sized chunk of China would probably be Japanese today and we might've had a Japanese-Indian war.

After WWII, The US had a unique viewpoint. Because we had just about every single culture on earth represented here, we could see how dumb these squabbles were. We had the bomb, and decided that instead of having a war every generation, we'd instead build the most terrifying set of ICBM's, Bomber planes, and nuke subs to make our adversaries know that attacking us or involving us in a war is Dumb. The tech for it was leaked to the soviets by US Scientists fearing a monopoly of power and M.A.D was born.

This shifted warfare into the use of economics and psychological warfare (cold war) to trash your enemy before an attack and the use of psychological warfare specifically and the creation of puppet ideologies used to weaken an adversary is what lead to the war against the spread of socialism\communism. E.G. The soviet union used ideological subjugation (look up Yuri Betzmenov on Youtube) successfully against countries including Afghanistan and Venezuala and those places are hot messed due to it today. Marxism is what we call the current puppet ideology in the US; it encompasses a lot of things the Russians do not do or believe in today and never really believed in and most people don't recognize that.

The Vietnam\Korean war was largely about stopping the spread of Communism by setting up military bases on Mainland china to keep an eye on the chinese, and creating capitalist\democratic economic powers in mainland China to serve as an example. It was a pre-emptive war to stop a bigger war from ever occuring.

The problem we have today with Conscientious objectors is psychological warfare is now a thing and if an adversary can eliminate your will to fight, they can take you by suprise or engage in domestic politics to create domestic problems to keep your attention at home.

In WWI, trench warfare was an excuse to send lots of perfectly good men to their deaths in a bullshit defensive game. Great thinking exercise; what is the cost per square mile of territory taken in human life? You'd extinguish the US male population before you got halfway into Germany. Given the survial rate of front line troops, objecting was an intelligent decision; that's no way to fight a war.

In WWII, they had less issues after pearle harbor due to the politics played. You had much better survivability and moral politics in that war.

When you get into the cold war and attacking smaller countries on the auspice of stopping the spread of an ideology, that is going to create cognitive dissonance.