The international system is an anarchical system: each State can resort to force if it believes appropriate, without thus infringing any code or convention whatsoever. What some people want to purport ( right term? ) as the closest thing to an International Legal Order, ignore that there is no WORLD Constitution, nothing like common values such as those written on a nat'l constitution. On top of that, most of the countries in the UN are non democratic so force is permitted.
Anyways I'd say half diplomacy and half strength in a general perspective: sumtimes 100 % diplomacy would be enough ( very seldom ), sum others 100 % would be ( less seldom, but seldom ).
I vote for "others" option. Until a shared common values- int'l order is established ( resembling a private State constitution ) there will never be peace on earth.