The Pentagon has commissioned military contractors to develop a highly classified replica of the Internet of the future. The goal is to simulate what it would take for adversaries to shut down the country’s power stations, telecommunications and aviation systems, or freeze the financial markets — in an effort to build better defenses against such attacks, as well as a new generation of online weapons.

The most exotic innovations under consideration would enable a Pentagon programmer to surreptitiously enter a computer server in Russia or China, for example, and destroy a “botnet” — a potentially destructive program that commandeers infected machines into a vast network that can be clandestinely controlled — before it could be unleashed in the United States.

Or American intelligence agencies could activate malicious code that is secretly embedded on computer chips when they are manufactured, enabling the United States to take command of an enemy’s computers by remote control over the Internet.

The advent of Internet attacks has given rise to a new term inside the Pentagon and the National Security Agency: “hybrid warfare.” It describes a conflict in which attacks through the Internet can be launched as a warning shot, or to pave the way for a traditional attack.