The Defense Advanced Research Projects Agency (DARPA) is a U.S. government agency. In the context of computing, DARPA is best known for developing the Advanced Research Projects Agency Network (ARPANET), which was a predecessor to the Internet.
Without the Cold War, the Internet may not have evolved into the system it is today. U.S. defense agencies and contractors conducted a great deal of initial research to build a decentralized network capable of surviving the loss of many of the underlying nodes. Some claim that the U.S. government wanted to build a network that could survive a nuclear attack, but whether or not this was the true motivation is debatable. The true motivation behind ARPANET was the unreliable nature of network links and a need to build a network that could facilitate access to the large supercomputers (of which there were very few), in the event of network disruption. Either way, reliability was key, and this spurred development in packet switching and related technologies that evolved into the system we know today.
As an interesting aside, DARPA was instrumental in the creation of today's Internet, as well as the use of the at (@) symbol in emails. Used to define usernames, the @ symbol was included in email addresses that connected to specific host names. This was patterned after a utilities' email designed for DARPA by Ray Tomlinson, an engineer.