Yeah, when reading about the collapse of the Roman republic recently I was struck by how unimportant the law (about not crossing the Rubicon with an army) was. For a long time, the law wasn't meaningful because nobody would think of bringing an army into Rome, it just wasn't done! Then eventually Sulla said "fuck this, I'm bringing in an army to enforce my will", and the law didn't do a thing to constrain him (or anyone who came after him). It seems to me that it was the social norms of Roman society which kept people from using military force to get their way, and that the law served no purpose except perhaps a very visible way to reinforce the social norm.