I don't think it's your imagination. I've noticed a change in tone, too. And as you say, with good reason.
This was an interesting analysis of an aspect I hadn't thought of before, that is, a division of the GOP into those particular two camps, the War Party and the Small Government Party. But I'd say that it is not new but has actually always been a constant in the GOP (people attracted by the idea of a strong national defense and people attracted by the idea of small government), although there have been different issues around which it has coalesced.
One of the features of the Small Government Party is a certain isolationist tendency, which, while it may sound good, is completely unrealistic for the US in today's world or any other. The War Party, having no concern for small government issues because it thinks small government might limit defense, attempts to conciliate the SGP by limiting wars so that it cannot be called imperialist or be accused of trying to spread US values beyond our borders. This leads to a tendency to fight half-hearted wars.
Practically as soon as we were a nation, we had to fight the Muslims (Barbary Coast pirates). But even in that case, we did what we are doing here: we didn't entirely defeat them. We knocked them back enough to get them to leave us alone.
The only people we thoroughly defeated were the Germans, and that was under a Democrat president. Curious issues to ponder.
Well, Japan too. But I take your point. The thing is (as I never learned in American history in high school or college) that in WWII, the left was more or less on America's side (fancy that!) because we were against Hitler, who committed the unforgivable sin of betraying the sainted "Uncle Joe."
I've got a foot in both of these "camps". Though I tend to wonder, why couldn't isolationism work? After all we tended isolationist from 1848 - 1898 and then from 1899 - 1940. Of course we would need to boost some technology development a bit.
Steyn is right, with so many people not believing in value such as individual right to self-determination and such at home, it ispointless to pursue it abroad.
"The only people we thoroughly defeated were the Germans"
Heh. Unfortunately, relying solely on Japanese histories of WWII provides less that a complete picture of the war's outcome.