Some philosopher came up with a concept of "Just wars", this was basically the Idea the some wars are Self Defense shootings on a national scale while others are more akin to a Mafia hit.
During the 20th century the US engaged in 5 Major wars, (WWI, WWII, Korea, Vietnam, and Gulf war I ) of those only 2 1/2 can be called "Just wars" WWI, WWII (in Europe ) and Vietnam were not just wars defending US territory, or vital interests abroad, they were politically motivated maneuvers aimed at gaining influence on the world political stage while under cutting our competitors. Neither Imperial or Nazi Germany posed any threat to the US, if they couldn't invade England how the devil are they going to invade US ? in 1939 / 1940 a large portion of the population, including such notable as Charles Lindbergh supported Germany and APPROVED of the Nazi regimes actions, (Google "American Bund" ) "Suffering of the Jews" ? Not likely when large numbers of Americans thought the same treatment appropriate for our own Jews and Blacks. The reason Nazi Germany had to resort to death camps was because none of the "goody 2 shoes" Democracies would accept them or allow them to emigrate to Palestine. ( from 45-49 the British waged open warfare against Jews in Palestine ) The Nazi's didn't care if the Jews lived or not as long as they were not in German Territory, when the SS Einsatz Gruppen performed mass executions of Jews in Eastern Europe the Locals watched and cheered, they willing turned in Jews for execution.
Morality and self defense played no part in our involvement, it was done so European investors would owe us
Under these circumstances it is necessary to convince your troops that the foe is evil incarnate, starting in WWI with the image of the "Hun" bayoneting babies and raping nuns, through the WWII image of Nazi world conquest, (Remember, Nazi Germany had no effective "fleet" only a few Commerce Raiders and "Pocket Battleships", the last German "Fleet action" was when they got clobbered at the 1916 battle of Jutland .Also they did not have the air power to attack us, the German air force could not even stage effective raids against Northern or Eastern England )
Yes it was pretty much BS, but without it Wilson and Roosevelt could not have raised the troops needed to make the difference in these wars which changed America from a nation where rich Europeans invested their money into a nation that invested billions of dollars in rebuilding Europe, as a side note it is interesting to note that the money that paid for the Marshal plan was the money European Countries owed us for war material. America paid for rebuilding Europe with European money.
We are not done with the Idea of fighting wars solely to gain influence in Europe either, the reason for US involvement in Vietnam initially was because the French threatened to quit the newly formed NATO (considering their record,the fact that their intelligence agencies were thoroughly penetrated by the Soviets and the fact that when they later did drop out NATO got along quite nicely I don't know why Ike cared, but he did) When the French finally had to admit defeat a combination of the Red scare and Arrogance convinced Our Generals that we could do better and save a 3rd world shithole "for democracy" over the next dozen years they found out that you can not defeat some one who refuses to give up. The only way we could have won against the North Vietnamese would have been to exterminate every person North of the DMZ, no other action would have broken their spirit of resistance. The Politicians and the public did not have the stomach for that because the foe had not been sufficiently demonized.
The 2 1/2 "Just" wars did not have a place in making my point but for the record they were,
The War against Japan in answer to Pearl Harbor and their flagrant attempt to drive us from the Pacific and the Asian markets was purely a self defensive war.
Korea, where we gained no markets that we did not already have but repelled an aggressive invasion of a peaceful nation
and Gulf War I, where again we did not gain but stood up for the little guy against the bully.