For all the ways that our political coalitions have changed over the last few generations — Southern Democrats joining the G.O.P., Northeastern Republicans turning Democrat, “Reagan Democrats” moving right, suburban Republicans voting for Joe Biden — there are patterns that persist across the generations.

That’s what we’re seeing in foreign policy right now, where Democrats and Republicans are dividing over Israel-Palestine and Ukraine-Russia, respectively, in ways that would have been familiar to the version of each party that existed 50 or even 75 years ago.

The Democrats, first, are replaying their Vietnam-era divisions in the split between the Biden administration and the pro-Palestine left. Again you have an aging Democratic president struggling to modulate a conflict with no certain endgame. Again his left-wing critics represent his party’s younger generation, their influence concentrated on college campuses, their power expressed primarily through disruptive protest tactics.

The language of the protesters is similar across the two eras, albeit with “settler colonialism” replacing “imperialism” as the favored epithet.

So is the internal dilemma of the left — namely, to what extent is it possible to oppose a military campaign against an insurgent force embedded in a civilian population without becoming dupes for the insurgency’s authoritarian (in Vietnam) or theocratic (in Gaza) politics?

So is the depth of the divide between progressives and the Democratic older guard — Cold War liberals then, liberal Zionists today — and the possibility that the debate will push some of the latter group toward a form of neoconservatism.