With the recent Russian-Ukrainian conflict I asked myself this question and I can't seem to find anything on google.
The Russian-Ukrainian conflict seems to be explained, rationalised and sort of toned down in the media, but isn't this typical for a conflict that could escalate into something bigger? Did Europeans walk around after Germany invaded Poland thinking it couldn't possibly lead to anything bigger?
So when did people stop and think "Shit, this is huge"? And what are some things we can look out for to predict escalation of the current conflicts?
It depends on your threshold for a World War. Britain and France had an ultimatum in place should the Germans invade Poland. So as soon as soon as Germany invaded Poland, UK & France vs Germany & Italy was on. Had there been no more aggression, the war probably would have been limited to Europe and North Africa.
As far as the two major expansions of the war after this, the Japanese entering the war and bringing in America wasn't a huge surprise, but the timing was. The US was embargoing Japan, and there was American support for the Chinese against the Japanese, things were headed for a confrontation.
I think everyone except the Germans were surprised when they attacked the Soviets. The Germans and Russians were considered nominal allies after the non-agression pact. All of a sudden as the Germans are consolidating a large European empire, they go after the Soviets.