At the battle of Carrhae in 53 B.C.
In the year 53 B.C., Marco Casio invaded Parthia with a 40,000 man army, and the goal of expanding the Roman Empire.
It was a disaster.
This was mainly due to the design of the Parthian bow, a weapon made with a laminated spring, with a range and power that made the Roman legions defenseless. 20,000 Romans died, 10,000 were taken prisoner.
The Parthians did not prevail because they had a better general, they prevailed because they had a better designer.
Excerpted from @Issue Journal of Business & Design.