I had to explain "cross the Rubicon" to my wife just yesterday, in fact. (Her English is good, but it's mostly everyday/around-the-house English
Is that just an English idiom? I'd assumed it would be known in most places where Roman influence was strong.
You would assume correctly. Has to do with our common Roman/European roots. It's not an original English idiom, nor is it an English idiom at all, really. Since modern English is a hodge-podge of borrowed words from several different cultures and root languages, and some ridiculous meldings ("television" for instance-half Greek, half-Latin) one could see how some could get confused. The great 20th century philologist, J.R.R. Tolkien, saw Icelandic as the closest thing to what Old English was. Modern English is a mess. It's ok, most people don't know what the words they're saying mean anyway so what difference does it make. I had some idiot the other day in a meeting try to make themselves look cool by using "bifurcated" to describe something that was split in three parts. [facepalm]