Barack Obama has taken up the cudgels of intervention as well, but with much more subtlety than his immediate predecessors. In Libya, for example, he both cultivated allied participation and limited the American role to combat support. Same with Mali. Even his drone attacks on the sovereign territory of other nations have come at a slow pace — only a few dozen have been launched this year — and with much stealth. Now he calls for the removal of Bashar al-Assad in Syria, but he has so far limited the notion of intervening to stepped-up support for “good rebels.” This is something like the position Ronald Reagan took with regard to arming the Afghan mujahideen in the 1980s — but that action is more properly labeled a “counter-intervention,” as there were over 100,000 Russian soldiers occupying Afghanistan at the time. Obama’s biggest test will come over Iran, where he could argue that self-defense compels intervention to prevent nuclear weapons proliferation. …

In the world of Star Trek, the Prime Directive served as a check, but not an unthinking ban, on intervention. And so there were many interventions and counter-interventions. My favorite was in the original series, the episode “A Private Little War,” when Captain Kirk ordered Scottie to make some flintlock muskets for the bow-hunting hill people of a planet where he had once tarried because the Klingons were arming the other side with these relatively advanced weapons. Kirk intervened, but only after much soul-searching, and in a proportionate way that established a balance of power and at least kept the Prime Directive in mind.