By 1982, the ARPANET was the largest of many distributed networks. The stage was set for a change in the role of ARPANET -- from research network to de facto backbone connectivitiy.
The growth in ARPANET and the need to interconnect networks led to the adoption of TCP and IP as the protocols for use on the ARPANET. The original ARPANET protocol, NCP, would only support 256 hosts; by August of 1982, 235 were already connected. Accordingly, on 1 January 1983 the DoD mandated that ARPANET transition to TCP/IP.
The transition to TCP/IP also facilitated a second change: the connection of external networks to the ARPANET. The Defense Data Network (DDN) had been created in 1982; the military portion of the ARPANET, dubbed the MILNET, was now separated from the remainder of the ARPANET and merged with the DDN. The MILNET was then linked with the ARPANET via gateways; CSNET and ARPANET were similarly linked as well, and others followed. The ARPANET became the major linkage between these other networks; traffic from nearly any host destined for another host on a different major network was thus likely to traverse a part of ARPANET in getting to its destination. ARPANET had become the first de facto backbone network.
Further details on this period of ARPANET's history can be found here.[2]