The following graph from the world bank website shows the world wide military expenditure in percentage of the world GDP over time from 1988 to 2015
I am trying to find similar data over a much longer period of time. Over the last 3000 years would be ideal! Of course, I realize that such data might result from very rough estimations and might eventually be specific to a certain area of the world and arguably not representative.
Where could I find similar data over a much longer period of time?