Acronym for Advanced Research Projects Agency Network, ArpaNet was a pioneering WideAreaNetwork funded by the US DepartmentOfDefense. It seems that back then, as today, American researchers could get more funding if they can somehow find a military use for their research. The basic premise (at least from the military angle) of ArpaNet was a communications network that could "route around damage".
It became operational in 1968 and served as the basis for early networking research, as well as a central backbone during the development of the InterNet. The ARPANET consisted of individual packet switching computers interconnected by leased lines. Protocols used include FTP and Telnet. It has now been replaced (for DoD use) by NSFnet and is widely acknowledged as the parent of the InterNet.