An invasion of armies can be resisted, but not an idea whose time has come.

Armies meet something solid to push against, and resistance to them is at least visible. An idea that has become plausible to enough people moves differently, because there is nothing to hold a line against; it spreads through ordinary conversation and is already inside before anyone organises. This is a claim about how change travels, not a claim that ideas are gentle. Plenty of them have arrived at terrible cost.

What it suggests practically is that arguing against something widely felt rarely works. If you want a shift, spend your effort on making the alternative thinkable to people rather than on defeating what they currently hold.