Reading Chesterton's commentary on America's entry into WWI, I was struck by his claim that America was a deeply pacifist nation. He uses this American reluctance to go to war as evidence of how egregious Prussian aggression was, not merely at violating Belgian neutrality but in its ongoing violation of other neutralities. Eventually even the Americans were drawn in, unable to ignore the injustice. The same might well be said of WWII. Our European allies, in fact, regarded our entry into the war as delayed unconscionably in the face of clear injustice.
We usually call this strain of American thought isolationism now. I had never seen it equated with pacificism. Yet I take the point immediately. In our history up until WWII, we showed ourselves quick to resort to skirmishes at need, but extremely reluctant to get dragged into full war. Even some things called "wars" by Americans did not involve anything like the full weight of the nation massed in battle. The Indian Wars were ongoing encroachments and skirmishes over many decades. The War of 1812, The Spanish-American War, the Mexican Wars were decidedly limited. Only the War Between The States was war in the sense that other nations know it - and it can be argued that it was so intense precisely because it was so long delayed. The Revolutionary War was mostly skirmishes at first, with many Americans reluctant to get dragged in.
We don't have that reputation now. We are regarded by many as overeager to go to war, involving ourselves where we have no business. Even many of our own citizens regard us as overeager, isolationists on the right who believe we should let the dang fool world get into whatever trouble it wishes, but without us - and the internationalists on the left who believe any assertion of our national interests interferes with the growth of international institutions. Yet the presence of both strains up to the present day argues for some continuity of the American "pacifism" Chesterton describes.
Since 1950, we have been involved in Korea, Vietnam, Iraq twice, and a number of skirmishes in Latin America and the Middle East, all of which have been criticised as unnecessary intrusions by America. (I will note in contrast that there are many conflicts we have stayed out of where a victim party wished very much for us to intervene.)
Well, what changed? Do we act differently, or is this mostly propaganda by our opponents, trying to achieve a free hand to oppress?
Both, and the key to understanding this is the internationalism of our enemies. From the time of the Bolshevik Revolution, and certainly during the Cold War 1945-1990, our enemy was not so much Russia or China as an hegemonous communism of a dozen flavors. These movements mutually supported each other in many nations. Rightly or wrongly, American was drawn in in much the same way it entered the World Wars - reluctantly, unwilling to commit full resources. We retained the same willingness to skirmish, but not to even officially declare most wars.
Now also, it is not so much individual countries we war against as an international Islamism, powerful but not universal even in the countries we have gone to war against. We are at war with parts of nations which show the same mutual cross-boundary support we saw in the USSR.
Both of these international movements are comparatively new things in the world. Worldwide, or even continentwide alliances were not much known before the 20th C. The closest equivalent was empire, uniting many tribes as a powerful force. But no one ever went to India to work there to spread the idea of the Roman empire; nor to Spain to advocate for the Mongol way of doing thngs; nor to Ohio to assist in the formation of a greater Austria-Hungary.
Interestingly, and with grim irony, Americans were one of the earliest examples of this spreading of ideas. Our revolution illustrated powerfully the virtue of self-determination. We set the idea out in the world, to conquer by pen instead of sword. Even earlier, Christianity spread by both sword and pen - but far more the latter. We created this world where ideas of how people should live and govern crossed borders without having to conquer.
There is a parallel here with the development of my own pacifism into some different attitude toward conflict. I remain a peaceful man, prefering to be left alone, prefering to see if something might be worked out with an antagonist. But like America at large, I have come reluctantly to the idea that this doesn't always work. There are people of real evil, unwilling to even entertain the idea that they might be wrong, and thus unwilling to stop aggressing. And that is in fact how we shall know them - there was never the slightest hint that Germany or Japan, China or Russia, Palestinians or Taliban were even dimly aware of any fault. There was never even a hypocritical acknowledgement that they might have overreacted or misunderstood. Their enormous certainty, contrasted with the tradition of the West, inherited more from Jerusalem than Athens, to doubt one's own goodness, is the dangerous pathology we war against. It is the attitude of criminals and sociopaths. Even the wildest anti-jihadist, the most implacable anti-communist, could always admit many things America had done wrong. Honest men can see at least some of their own sins; our enemies have generally been those who can see none.