World War 1 (I hate Roman numerals) is considered to have lasted from 1914-1918; World War 2 is considered to have lasted from 1939-1945; but when did the world (historians, politicians, newsmen, etc., then people in general) realize that world wars had begun in 1914 and 1939? I doubt it was when Ferdinand was shot or Germany invaded Poland. At some point in the future, the clarity provided by hindsight pinpointed those years and events as the start of "something big."
I truly want to know the answer to these questions, but what prompts them is the thought that perhaps Russia's invasion of Ukraine was the beginning of World War 3.
Actually, these World Wars may have been World Wars 2 through 4, if the Seven Years War of 1756-1763 (which is described as "a global conflict") was the actual first World War, but that's a different conversation.