The problem is that everything is always changing, but that we necessarily try to predict the future based on our experience of the past.
Until the 20th Century, the geographic position of the United States meant that we were secure, without a large standing army. Plus we had the historical luck that after 1815, Europe was largely at peace for a century.
The 20th Century was different. Europe's evolution towards an ever-more-democratic, ever-more-peaceful society -- which we can see today -- was radically disrupted by the First World War, which destoyed three empires and gave birth to a powerful Communist, and then a powerful Fascist, presence in the world. We spent the rest of the 20th Century trying to recover from their destructive effects.
The United States had a close call with respect to WWII. Our President then knew we had to get involved in it, but had a very unwilling nation behind him. Thank God we were able to maneuvre the Japanese into firing the first shot, so that the nation could be rallied for war, and additional thanks for the fact that Hitler was mad enough to declare war on us, which he did not have to.
Then we had to fight the Third World War ... a Cold one, mostly. Close calls there, too, as in Korea. And the Cuban Missile Crisis could have gotten very nasty, as we now know. Fortunately, socialism is a terribly inefficient system, so the USSR finally collapsed under its own weight.
Now we face new challenges -- radical Islam, Russia, China. How tempting to think we could go back to the 19th Century situation, and just let the rest of the world fight it out.
But we can't. Not only that, but the nature of modern weapons means we have to do our feeble best, in a very murky situation, which we don't understand well and which is changing all the time, to nudge the world towards becoming the sort of stable liberal democracies which generally do not go to war against each other.
No one really knows how to do this, or if it is even possible. In any case, we had better remain militarily strong.
It is a paradox, in a way. I think Maineman sees -- as I do -- that, looking at human history in the long run -- our species may be entering a new epoch, in which the whole world gradually becomes like Europe, with war a thing of the past. But if this is becoming true, it is only doing so very slowly -- the rest of this century is not going to be a peaceful one -- at best an armed truce.
So we have to keep our powder dry, and keep a lot of powder handy. Butter isn't all that good for you anyway.