theriddlen said:
Machine guns for every nation.
That would really stop their wild expansions with because of being totally defenseless against these.
People would settle down, stay at the cities and build social structure, work on the ways of human kind.
Instead of hundreds years of senseless power struggle and fear - progress.
I'm pretty sure that was part of Richard Gatling's theory too when he invented the Gatling Gun! That and reducing the number of participants in a war and thus saving lives. To quote...
"It occurred to me that if I could invent a machine ? a gun ? which could by its rapidity of fire, enable one man to do as much battle duty as a hundred, that it would, to a large extent supersede the necessity of large armies, and consequently, exposure to battle and disease [would] be greatly diminished"
Unfortunately whilst Gatling knew how to divide, the Generals knew how to multiply, changing the face of war and politics forever.
To answer the question, if I would give anything to the ancients it would be knowledge. Science, Maths, advanced metalurgy, medicine, perhaps even a 'universal' alphabet and language, these things would be of greater benefit than any single object or machination of modern society.