So just for fun, today I plugged in the terms "public diplomacy" and "soft power" and traced them from 1970 to 2008, the last year for which Ngram has results. See the chart below (and yes, I do realize it's difficult to read. You can see the original here).
And check out the results for diplomacy v. propaganda, over a larger time span:
And here are some interesting parallels between the rise and fall of democracy and propaganda's popularity: