The gates to the Temple of Janus in Rome were closed in times of peace and opened in times of war. As you can imagine, they were open a lot. Caesar Augustus bragged that before his birth the gates had only been closed twice in the 800 years since the founding of the city, but during his reign he claims to have closed them three times.

I think that the open gates fits the bill for an 'abstract symbol associated with war'. I know you probably meant a written symbol, but this is the only one that I know of.