In the earliest days, this was a project I worked on with great passion because I wanted to solve the Defense Department's problem: it did not want proprietary networking and it didn't want to be confined to a single network technology.
Choosing a single most important development is incredibly hard to do because a lot of different things had to happen before the Internet could be deployed in the fashion it is today.
While many governments are committed to maintaining flexible regimes for fast-moving Internet technologies, some others have been quite explicit about their desire to put a single U.N. or other intergovernmental body in control of the Net.
In 1970, there was a single telephone company in the United States called AT&T, and its technology was called circuit switching, and that was all any telecom engineer worried about.